Adult literacy skills are critical assets of individuals as well as societies in terms of economic (e.g., human capital) and social well-being. Thus, it is important to monitor the long-term trends and sources of skill gain/loss. Yet, temporal sources of adult literacy skill variation are understudied. The current study analyzed three comparable, nationally representative datasets including the 1994 International Adult Literacy Survey; the 2003 International Adult Literacy and Life Skills Survey; and the 2012/2014/2017 Program for the International Assessment of Adult Competencies. Results from the age-period-cohort hierarchical cross-classified random effects model showed significant variation in adult literacy skills across cohorts (1930-1995) and time periods (1994-2017). Possible implications for adult education policies and future research needs were evaluated.