The study explored grade 12 students' understanding of reaction kinetics, a topic which has not been extensively explored in the chemistry education literature at this level. A 3-tier diagnostic instrument with 11 questions was developed--this format is of very recent origin and has been the subject of only a handful of studies. The findings reveal that reaction kinetics is not an easy topic for the students to understand. A total of 23 alternative conceptions (ACs) as well as some indication of their strengths and other confidence-related measures have been documented for the students (N = 137)--most of these ACs have not been reported before. When the overall scores of the students in the diagnostic test are ranked, it was found that there are a number of differences in the ACs held by students in the upper and lower 25% of the sample. While most of the ACs held by these groups are common to the overall sample, a number of these are not found in the overall sample. The findings further underscore the diagnostic utility of the 3-tier format. Some implications of the findings are discussed.