Background/Context: The landscape of college remediation programs experienced significant shifts from prerequisite models to corequisite models in the past few years across the nation. The traditional prerequisite models required students placed below college level to pass a sequence of remedial courses before they could enroll in college-level coursework; many studies found these programs actually hindered students' progression towards college degrees (e.g., Bailey, Jeong, & Cho, 2010; ScottClayton, Crosta, & Belfield, 2014). In response to studies highlighting the poor outcomes of prerequisite remediation, more than 20 states have implemented corequisite approaches, allowing remedial students--and in some cases mandating everyone regardless of the level of academic preparation--to take college-level courses with concurrent academic support upon initial enrollment. Early studies on the effectiveness of corequisite remediation showed promising results, particularly for gateway course outcomes (e.g. Logue, Douglas, & Watanabe-Rose, 2019; Meiselman & Schudde, 2020; Miller et al., 2022; Ran & Lin, 2022). Purpose/Objective/Research: Question This study has two objectives. First, we aim to examine the heterogeneous effects of corequisite versus prerequisite remediation on college success by prior academic preparedness. Second, we intend to understand the mechanisms of the differential impacts of corequisite remediation across student populations. The first research question addresses an important gap in the literature on college remediation. Namely, much of the rigorous research on traditional prerequisite models relied on regression discontinuity methods; these studies identified "local" effects using students just above and below college-level to establish causality (e.g., Martorell & McFarlin, 2011). It is still unclear what are the implications of enrolling everyone into college-level courses right after initial enrollment, especially for those with placement scores further away from college-level. The second research question will provide important insights into the implementation of remedial reform. Some recent research started to look into different components of remedial reform. For example, Kane et al. (2021) decomposed the impacts of remedial programs into the positive developmental effects and the negative delay/displacement effects. It is likely that the developmental effects and the delay effects under corequisite remediation are variable for students with different levels of prior academic preparation. Weakly prepared students with lower placement scores may experience a larger developmental effect, because they benefit from intensively focusing on building basic academic skills (Scrivener et al., 2018); moderately prepared students, who just missed the college-level cutoff by a few points, may experience stronger negative delay effect, because they could have performed just as well in college-level courses even without remedial courses (Scott-Clayton et al, 2014). By unpacking the mechanisms, researchers, policymakers, and practitioners will understand what reform components work, and for whom. Setting & Population/Participations/Subjects: We obtained administrative data for close to 100,000 students enrolled at any TBR community colleges between academic years 2010-11 to 2019-20. TBR was the first system across the nation to mandate and scale corequisite reform in 2015. Intervention/Program/Practice: TBR piloted corequisite remediation in 2014 and scaled it up system-wide in fall 2015. In Tennessee, students are required to take a college entrance exam (typically ACT) during the 11 grade. They are notified of their remedial status during high school. Those who enter a TBR community college with ACT scores below 19 for math or reading and below 18 for English are required to take remedial courses. Under the corequisite models since 2015, these students were able to enroll in college-level math or English courses with a concurrent corequisite learning support course. Our analyses of student transcript data suggest that virtually no students took standalone prerequire remedial courses after the corequisite models were in effect. Research Design & Data Analyses: We apply a difference-in-differences model, exploiting that any change in remediation policy should affect only students below college-ready level. We compare the academic outcomes of remedial students before and after the implementation of corequisite models, while using college-ready students as the control group to adjust for any general time trend or policy change that potentially affected all students. In this model, we also control for cohort fixed-effects, high school-by-cohort fixed effects, and a set of student covariates. To capture the heterogeneous effects of corequisite remediation by academic preparedness, we estimate a difference-in-difference model with interaction terms of different placement score bins. The coefficients of the interaction terms between placement score bins and post-reform indicators capture the effects of placing into corequisite versus prerequisite remediation for students with different levels of prior preparedness. Findings/Results: Our preliminary results are summarized as follows. First, corequisite models were overall more effective in helping students pass the first college-level English and math courses, compared with the traditional prerequisite model. Second, for math, moderately prepared remedial students experienced larger benefits from corequisite reform, compared with weakly prepared remedial students. First-year gateway math completion rates increased by 21 percentage points for students who scored right below the college cutoff (with ACT between 16 to 18), while the rates improved by 11 percentage points for those with the lowest placement scores (with ACT math below 12). For English, the patterns were reversed, students with the lowest placement test scores for writing and reading experience up to 35 percentage points improvement in first-year gateway English completion rates, while the effect for those right below the college-level was around 18 percentage points. Lastly, students who were placed into corequisite remediation enrolled up to 4 more college-level credits, compared with their counterparts placed into prerequisite models, but we did not find any significant impacts on credential completion three years post initial college enrollment.