This contribution is based on an exploratory study aiming to analyse the professional identity-building processes in which young students are involved as part of a university-level Co-operative Education program for training educators. The empirical plan for the analysis focuses on a body of fifteen "professional development portfolios" written by these students. The conceptualization of what these writings provide as a basis for analysis is based primarily on the hypothesis that their formative impact is based on a review, in the university context, of the internship experience completed. The Co-operative Education system thus corresponds to two movements with respect to the history of each student: the internship itself and the confrontation with formalized knowledge allowed by the portfolio. The objective of this article is twofold: to try and understand which ingredients of the students' professional development processes the writing guidance is likely to influence in a university context, and beyond that, to identify avenues in the professionalizing and formative aspects of the guidance area in Co-Operative Education systems.