Academic peer leadership positions provide opportunities for undergraduate students to develop content knowledge, Twenty-first Century Skills, and their beliefs about teaching and learning. To explore peer leaders' (PLs') epistemological development, the Epistemological Beliefs about Physical Sciences (EBAPS) survey was administered to 135 PLs three times a year, over a three-year period. This instrument was demonstrated to be valid and reliable for use with STEM PLs. Although the majority of positive shifts within instrument constructors occurred during the fall semester, responses did not regress by the end of the spring semester, demonstrating a retention of the new or altered beliefs over time. Implications for the design of PL development programs are discussed.