This study is designed to test a reciprocal causation, cross-lagged model of self-concept, self-efficacy, and achievement in a postsecondary STEM course. Both self-efficacy and self-concept are known to be related to achievement; however, there is a need to untangle the relationship between the two constructs as well as their association to achievement across time to best direct future research efforts. To achieve this research interest, a longitudinal measurement strategy was used to measure chemistry self-concept and self-efficacy for learning and performance before and after achievement measures (i.e., two term examinations) in a postsecondary organic chemistry course context. A reciprocal causation, cross-lagged model best fits the data as a representation of the relationships between these three measures over time as compared to autoregressive, performance effects, and self-belief effects models. Significant paths in the reciprocal causation, cross-lagged model include the first self-concept measure to the first achievement measure as well as from the second self-concept measure to the third self-efficacy measure. Relationships from achievement to each subsequent self-belief measure were also significant. This study demonstrates the ability of longitudinal measurements of multiple constructs in postsecondary STEM educational research to collect nuanced information that is overlooked when pre-measure designs of single constructs are used. In the classroom, an initial measure of self-concept can inform instructors of the likelihood of students to succeed on an initial achievement measure, at which point they may choose to implement some of the targeted intervention strategies from literature.