Drawing on role theory and socio-constructivist ideas about learning, this study explores how peer-tutoring can support tutors' learning. The sample comprised ten 16-17-year-old biology tutors, working with twenty-one 14-15-year-old students from a science class over eight weeks. Data were collected through an online wiki, tutor interviews, paired tutor discussions and video recordings. Tutors' perceptions of their role motivated them to learn the material, and their learning was supported by discussion and explanation, revisiting fundamentals, making links between conceptual areas, testing and clarifying their understanding, and reorganising and building ideas, rehearsing them, and working through them repeatedly, to secure their understanding. When tutors employed long answer questions, there was evidence of reflection on their learning and links made between conceptual areas. When preparing to tutor, tutors could focus on key points and engage with basic ideas from alternative perspectives. Mental rehearsal of peer-tutoring episodes helped them appreciate weaknesses in their own subject knowledge.