Background: Computer-supported collaborative learning (CSCL) environments are hypothesised to offer a learning environment that satisfies basic psychological needs for autonomy, relatedness and competence, subsequently improving learning and motivational outcomes. However, the underlying mechanism of how basic psychological needs are fulfilled during the collaboration process is unclear, and little is known concerning group size differences in collaboration and motivational outcomes. Objectives: We examined the role of peers in collaboration and whether peers support or thwart each other's needs via their interactions. Methods: Data were collected from 155 undergraduate students in an experimental study, in which they were randomly assigned to work in dyads or quartets. Results: Analysis of peer interactions revealed a low number (< 10%) of thwarting interactions. Dyads and quartets showed similar frequencies of need-supportive behaviours (around 30%-40%). In contrast to expectations, peer interactions did not predict the fulfilment of basic psychological needs. Group size did predict need satisfaction: dyads reported more autonomy and relatedness satisfaction compared to quartets. Groups who--on average--experienced more autonomy satisfaction, but less relatedness satisfaction performed better on the individual post-test. However, need satisfaction was not associated with group performance. Conclusions: The present study is among the first to investigate peers as providers of need support in the context of collaborative learning; future research is urged to further investigate and develop this framework to improve the quality of peer communication and group performance in CSCL.