This paper examines the role of prior knowledge in first year performance of undergraduate chemistry, considering specifically the context of general student aptitude. Statistical testing shows that there were significant differences between the mean scores of students who have and those who have not prior knowledge of chemistry in semester tests and end of year 1 exams, with the former group obtaining higher scores. Correlational analysis shows a strong correlation between prior knowledge and exam performance, and allows for probing of the role of student aptitude. Finally, regression analysis confirms that prior knowledge has a demonstrable influence on future exam performance over and above student aptitude. Two prediction models based on the regression analysis study are proposed. (Contains 4 tables and 1 figure.)