This study aims to investigate the role of mediation in the learning process from a sociocultural perspective, activity theory in particular. This study was carried out in a primary English immersion school within the CCUEI Programs in Mainland China. Data were collected mainly through observations and interviews, which were then supplemented by field notes. Techniques of discourse analysis were used to analyze the data. The recorded peer talk presents an emic picture of what occurred during children's interactions. Research findings show that mediation includes task, teacher talk, peer talk, subject content, semiotic artifacts and other semiotic tools. Peer talk as well as linguistic resources constitutes an important resource aside from teacher talk, where the subject content serves as an important mediational tool in the activity and the content-based immersion programs, while offering students opportunities of using the language in meaningful interactions. The results offer insights to both students and teachers about language learning and about pedagogy and professional education respectively.