This study explores the relationship between different types of prior knowledge and student achievement in an introductory chemistry course. Student achievement was regarded as the pace of completing the course as well as the final grade. A model of prior knowledge is proposed; this distinguishes between different types of prior knowledge and suggests how different types of prior knowledge should be assessed. The participants were 193 chemistry students from the University of Helsinki. Their prior knowledge was assessed with a questionnaire based on the prior knowledge model. The results indicate that the quality of prior knowledge is clearly reflected in the pace of completing the course and in the tendency to drop out of the course. Students who had deeper-level prior knowledge were more likely to complete the course in the pre-scheduled time and to get higher final grades, whereas students who performed lower in the prior knowledge test were more likely to either drop out or did not complete the course in pre-scheduled time. The study implies that making a distinction between different types of prior knowledge is a potential way to identify students who are in need of more support. (Contains 6 tables and 1 figure.)