While CLIL programmes have been extensively researched for their impact on L2 learning outcomes, the relationship between young learners' L2 proficiency and subject knowledge has received less attention. This study aimed to address this research gap by examining the science content knowledge of two cohorts of Grade 5 students (n = 100) from two primary schools in China, with one school implementing a CLIL science programme and the other offering a conventional science programme. The study also investigated the correlation between science achievement and L2 proficiency, measured by assessing learners' English speaking, listening, reading, and writing skills using the Cambridge Young Learners English Tests. Science knowledge was evaluated with a tailored test aligned with the National Science Curriculum. Results indicated no statistically significant differences in science achievement between CLIL and non-CLIL students, with the former achieving higher mean scores. We also found a significant positive correlation between science knowledge and overall L2 proficiency. Notably, receptive language skills significantly correlated with science scores, while productive language skills did not. These findings provide positive evidence for the potential effectiveness of CLIL programmes in the primary education setting in China.