The present study examines the relative role of three distinct types of peer relationships (reciprocated friendships, frequent interactions, and shared group membership) in within-year changes in academic self-concept and engagement before and after the transition to middle school (fifth and seventh grade). In a series of linear regression analyses, main effects of each peer type's academic self-concept and engagement on changes in youths' academic characteristics were used to test socialization processes. Interactions of youths' academic skills with those of each peer type were used to test social comparison processes influencing changes in academic self-concept. Results suggest unique roles of each peer relationship differentially influencing changes in youths' academic adjustment as well as stronger influence effects during seventh than fifth grade. Implications are discussed in terms of distinct influence processes associated with each peer relationship type as well as potential developmental differences in the role that certain peer relationships play. (Contains 3 tables and 1 figure.)