Compared to studies on school students' understanding of various topics in the sciences, studies involving university students have received relatively less attention in the science education literature. In this study, we investigated university students' understanding of transition metals chemistry, a topic in inorganic chemistry, which has been only scarcely explored in the science education literature. A four-tier diagnostic instrument was used. The instrument comprises 25 questions, and each question has an answer tier, a confidence rating for this tier, a reason tier and a confidence rating for this tier. Versions of the instrument were refined iteratively during the preliminary and pilot phases of the study. This study reports on the results obtained from the main phase of the study, using a sample of 140 students. Overall, the diagnostic test was difficult for the students. The students had a mean score of 38%, based on correct responses for both answer and reason tiers for the questions. It was accompanied by a mean confidence of only 3.49 out of 6 (that is, 58.2%) for the whole test. The results indicate that transition metals chemistry is a difficult topic for the students. Twenty-four alternative conceptions have been identified in this study, including some indication of their strengths. Some implications of the study are discussed.