This study focused on grade 12 students’ understanding of reaction kinetics. A 4-tier diagnostic instrument was developed for this purpose and administered to 137 students in the main study. Findings showed that reaction kinetics is a difficult topic for these students, with a total of 25 alternative conceptions (ACs) being uncovered. Except for one AC, the other ACs uncovered have not been reported before in the literature. An interesting point emerging from this study is that nearly 70% of the ACs were obtained from questions that featured graphs. Overall, the 4-tier format for the diagnostic instrument demonstrates good utility for probing students’ understanding of reaction kinetics as well as uncovering their ACs. The confidence-related measures, which are more commonly used in the educational psychology literature, have also permitted further insights to be gained into how the students performed in the test as well as the classification of the ACs.