This study investigates how peer assessment can help students to learn about photosynthesis in a "high attaining," year nine class in a UK 11-18 comprehensive school. There is limited research on how peer assessment can influence the learning of "high attaining students"; most existing research focuses on how formative assessment can benefit learning and self-esteem in low-attaining students or those with learning disabilities (Black and Wiliam, 1998a). A conceptually demanding topic like photosynthesis provides an excellent context to examine how peer assessment can impact on the learning of high attaining students. Analysis of data derived from questionnaires, individual interviews and conventional tests suggested that (1) a six week multi-component peer assessment intervention can have an impact on students' learning, and (2) some of the effect on learning may be linked to the way in which peer assessment enhanced students' own understanding of the role they play in their learning, and hence their autonomy in learning. (Contains 1 figure and 2 tables.)