Research experiences for undergraduates (REUs) seek to increase the participating students' knowledge and perceptions of scientific research through engagement in laboratory research and related activities. Various REU outcomes have been investigated including influence on participants' content knowledge, career plans, and general perceptions of their domains of research. The complexity of REUs and dynamic nature of student development provide opportunity for exploring how REUs influence student growth. Our research focused on first- and second-year college students who participated in a residential REU program that took place in a chemistry department in a metropolitan university in the western United States. We assessed the standard REU outcomes and sought to document the emotions the student experienced through their participation. In addition, we used the developmental framework of self-authorship (Baxter-Magolda, 2004) as a lens to investigate the participants' professional identity development. Our mixed methods research revealed shifts in the participants' perceptions of science, increases in their knowledge of chemistry, and clarity in their career trajectories. We also found that the REU participants experienced profound levels of professional identity growth and used a number of affective terms, such as confidence, persistence, patience, and enjoyment, to describe their experience. Interpretations and implications are discussed.