Although the benefits of collaborative learning across disciplines are well-established, its effectiveness hinges on the quality of execution. Motivational beliefs and emotions are crucial in students' engagement and achievement. Yet, a notable gap exists in examining these variables among students with varying proficiency levels in the context of collaborative learning. To address the gap, the present study investigated 289 fourth- and fifth-grade Hong Kong primary students' motivational beliefs and emotions in ESL collaborative learning and explored the variation across students' actual and self-perceived English proficiency. The findings revealed an overall high level of motivational beliefs alongside a medium level of anxiety. Structural equation modelling analyses indicated that self-perceived proficiency positively influenced students' motivational beliefs, whereas actual proficiency did not make a unique contribution. Both self-perceived and actual proficiency, as well as their interaction, reduced anxiety. Actual proficiency had a stronger negative impact on anxiety when self-perceived proficiency was high, and the influence of self-perceived proficiency was significant only when actual proficiency was high. Practical implications are discussed.