This study used the Programme for the International Assessment of Adult Competencies (PIAAC) prison sample to explore the predictive contributions of Print Vocabulary and Sentence Processing skills to the Literacy and Passage Comprehension performance of 461 U.S. prisoners. Both predictors were significant, but Print Vocabulary accounted for less than 1% unique variance. Nine demographic and prison-specific activities were explored as moderators of the relation between Sentence Processing and Literacy or Passage Comprehension. For the literacy outcome, only learning disability status, age, educational attainment, and job prior to incarceration significantly moderated the Sentence Processing--Literacy relation. For the Passage Comprehension outcome, only age, job prior to incarceration, prison library use, and prison educational attainment moderated the Sentence Processing--Passage Comprehension relation. Incarcerated adults who rarely or never used the prison library and had low Sentence Processing scores exhibited the weakest Passage Comprehension performance. Differences in moderation results may be a function of the manner in which PIAAC assessed reading skills. The findings may have important implications for tailoring correctional education to specific demographic characteristics and prison-specific activities as well as improving inmates' post-release success.