Studies have shown that the Study Process Questionnaire (SPQ)--which provides a measure of student approaches to learning--is a relatively weak predictor of academic achievement. The present study sought to explore whether students' achievement-related classroom behaviours, as observed by teachers, can be used as a mediator between student approaches to learning and academic achievement. The SPQ was administered to 1,608 students enrolled in six different diploma programmes offered by a polytechnic in Singapore. Data were analysed by means of correlation and path analysis. In line with existing studies, the results revealed that student approaches to learning was a weak predictor of academic achievement. However, achievement-related classroom behaviours turned out to be a significant mediator between student approaches to learning and academic achievement, effectively doubling the explained variance in academic achievement. Implications of these findings for using the SPQ are discussed. (Contains 1 figure and 2 tables.)