Background: Socioeconomic gaps in children's academic and social skills are large upon Kindergarten entry (von Hippel, Workman, and Downey 2018; Reardon 2011; Reardon and Portilla 2016). Preschool programs may yield particularly large benefits to low-income students and racially minoritized groups and may be promising for reducing inequalities (Bassok 2010; Gormley and Gayer 2005; Weiland and Yoshikawa 2013). Yet center-based three- and four-year-old preschool, while expanding, remains largely unequal in access and enrollment (Bassok et al. 2016; Kulic et al. 2019; Magnuson and Waldfogel 2016). This paper will decompose inequalities in academic and social skills upon school entry into the share due to enrollment in center-based preschool (and thus subject to intervention by expanding access and enrollment) and the share due to gaps in family income and maternal education. A growing body of work has estimated preschool's role in reducing income and race-based disparities on academic outcomes. Using population projections of effects from high-intensity, targeted programs, studies find universal or targeted preschool programs have potential to yield large reductions in racial/ethnic and income-based skills gaps among school-aged children (Duncan and Sojourner 2013; Magnuson and Waldfogel 2005). Further, studies using seasonal comparison designs to estimate skill gains during the school year relative to summer months find equalizing or neutral effects on income-based gaps in literacy, numeracy, and social-emotional skills (McCormick et al. 2021; Thompson et al. 2023). Finally, one recent metanalysis of early childhood programs across 13 countries finds that universal programs can have equalizing effects on children's outcomes throughout their lives (Schmutz 2024). This study will use a different estimand and estimation method for identifying the contribution of enrollment in preschool to socioeconomic inequalities (Jackson and VanderWeele 2018; Lundberg 2022). Using two nationally representative, longitudinal datasets, I will identify the residual disparities in reading, math, and social skills based on family income if enrollment in center-based care were equal across students of high- and low-income groups. I will compare this with the residual disparity if enrollment in center-based care and the distribution of maternal education were equal across groups. This offers several advantages and complementarities to existing studies. First, I use observed differences in skills of a representative sample of students in the US. Second, I investigate the residual disparities due to enrolling in any center-based preschool relative to high-quality center-based preschool. Third, decomposing the effects of family income and maternal education isolates two sources of inequalities that operate both jointly and separately. While maternal education is highly predictive of family income, it is independently associated with enrollment in high quality early childhood programs and children's early academic skill formation (Augustine, Cavanagh, and Crosnoe 2009). Research questions: 1. If we intervene to equalize enrollment in center-based three- & four-year-old preschool, across income quintiles, what would be the residual disparity in school entry skill-levels? 2. If we intervene to equalize enrollment in center-based three- & four-year-old preschool and the distribution of maternal education, across levels of income quintiles, what would be the residual disparity in school entry skill-levels? I look at each of these questions first, based on enrollment in any center-based preschool classroom and second, based on enrollment in high quality classrooms. Data: I use the Early Childhood Longitudinal Study -- Kindergarten (ECLS-K) and birth cohort (ECLS-B), nationally representative samples of [approximately]14,000 children in Kindergarten (2011) and [approximately]7,000 children beginning at nine-months old through Kindergarten (2001-2007). I use student assessments at Kindergarten in language and literacy, mathematics, and motor skills to measure income-based disparities, I compare the highest quintile income group and the lowest income quintile group. For maternal education, I use years of education. I construct quality measures using -- a subscale of the Early Childhood Environment Rating Scale. Analytic models: Following Jackson and VanderWeele (2018), where Y is a child's outcome at Kindergarten entry, X is enrollment in center-based care, I is a child's family income, and E is maternal education, I estimate: [equations omitted]. Under an intervention to equalize preschool enrollment across low- and high-income children (RQ1), the residual disparity in children's outcomes would then be equal to: [theta][subscript 1] + ([theta][subscript 2]/[gamma][subscript 2])([phi][subscript 1] - [gamma][subscript 1]) and the disparity reduction would be ([gamma][subscript 1] - [theta][subscript 1]) + (1 - [theta][subscript 2]/[gamma][subscript 2])([phi][subscript 1] - [gamma][subscript 1]). Under an intervention to equalize preschool enrollment "and" maternal education (RQ2), the residual disparity in skills equals [theta][subscript 1] and the disparity reduction equals [phi][subscript 1] - [theta][subscript 1]. I estimate these disparities with and without covariates (C): student race/ethnicity and whether English is spoken at home. I bootstrap standard errors. Findings: Consistent with prior studies using ECLS-K, 70% of children with families in the highest income quintile enrolled in center-based care in the prior year, whereas this was true of 45% of children in the lowest income quintile. Children in the highest income quintile enter Kindergarten scoring 0.98 standard deviations higher on reading assessments and 1.07 standard deviations higher on math assessments than children in the lowest income quintile. Initial findings using ECLS-K presented in Table 1 suggest equalizing enrollment in any center-based preschool would reduce income-based inequalities in reading by 0.03 sd (se = 0.004 ) or 4% and math by 0.02 sd (se = 0.003) or 3%. Equalizing enrollment in center-based care and maternal education would reduce gaps in reading by 0.38 sd (se = 0.02) or 50% and in math by 0.36 sd (se = 0.02) or 47%. Results using ECLS-B adjusting for classroom quality are in progress. Conclusions: Initial findings suggest while equalizing enrollment in center-based care may reduce disparities in Kindergarten skills by a small proportion, these disparity reductions are dwarfed in comparison to also intervening on maternal education. Expanding access to and encouraging enrollment in preschool alone is not enough for substantial disparity reductions upon Kindergarten entry. Other strategies tackling income inequality and disparities in families' social resources, as well as ensuring quality experiences in early childhood programs are critical in reducing income-related gaps in school readiness. Forthcoming analysis will address the latter, while additional research should investigate ways in which two-generational approaches may further reduce inequities in young children's lives.