Fundamentally concerned with motivation, self-determination theory (SDT) represents a framework of several mini-theories to explore how social context interacts with people's motivational types categorized by degree of regulation internalization. This paper aims to modify an existing theory-based instrument (Academic Motivation Scale, or AMS) and provide validity evidence for the modified instrument (Academic Motivation Scale-Chemistry) as a measure of seven types of student motivation toward chemistry. The paper explores how motivation as measured by AMS-Chemistry is related to student academic achievement and attendance. In a pilot study, the unmodified AMS showed good reliability, reasonable data fit, and the ability to detect motivational differences by sex in college chemistry courses. Based on the pilot study results, expert panel discussions, and cognitive interviews with students, the Academic Motivation Scale-Chemistry (AMS-Chemistry) was developed. AMS-Chemistry was administered to university students in a first semester general chemistry course twice within a semester. An examination of validity evidence suggested that the AMS-Chemistry data could be used to investigate student motivation toward chemistry. Results showed students were extrinsically motivated toward chemistry on average, and there was an overall motivational difference favoring males with a medium effect size. Correlation studies showed motivation was not associated with academic achievement at the beginning of the term, but intrinsic motivation subscales ("to know," "to experience," and "to accomplish") were positively associated with academic achievement at the end of the term. Results also showed that students who persisted in class attendance scored higher on intrinsic motivation subscales than those who did not persist. The 28-item AMS-Chemistry is easy to administer and can be used to better understand students' motivation status and how it might change across the curriculum. Faculty interested in promoting student intrinsic motivation may also use the AMS-Chemistry to evaluate the impact of their efforts.