This article investigates practices that engage preservice teachers' ideas of professionalism within the context of a university-based mentoring programme. The case is explored by observation of mentoring sessions, analysis of documents grounding the mentoring sessions, and mentor and mentee interviews. Practices are investigated through the theory of practice architectures and made the subject of a thematic analysis. The purpose of the investigation is to highlight how practices can contribute to enhancing preservice teachers' understanding of different aspects of professionalism, particularly those related to expectations of teachers' professional competence. The findings show how mentoring practices can contribute to widening preservice teachers' understanding and how contextual factors, like power relations, can come into play in mentoring practices. The analyses illuminate how professionalism is being negotiated in a third space institutionalised in a campus-based mentor programme in teacher education.