Peers carry potential in enhancing students' self-assessment development, but few studies have explored how peer scaffolding is enacted in the process. This qualitative study explores peer assessment effects on the self-assessment process of 11 first-year undergraduates and the factors limiting peer influence. Drawing on the data from students' journals, follow-up interviews, observations of in-class formative peer assessment activities and teacher interviews, we ascertained that peers could aid the self-assessment process by enriching student understanding of quality, refining subjective judgement and deepening self-reflection. Yet, peer influence could be reduced by distrust, tensions in feedback communication, competition and lack of readiness for peer learning. Implications for effective use of peers in supporting self-assessment development are discussed.