The results from a study to explore pre-service teachers' understanding of ionization energy, a topic that features in A-level (grade 11 and 12) chemistry courses. in Singapore , is described. A previous study using a two-tier multiple choice diagnostic test has shown that Singapore A-level students have considerable difficulty understanding the topic of ionization energy. The same diagnostic test was administered to a total of 237 pre-service chemistry teachers from 2003 to 2006, and it was found that the pre-service teachers had similar difficulties and alternative conceptions as the A-level students. Teachers need to be aware of their students' as well as their own alternative conceptions so that they can address their subject matter knowledge and be better able to support their students' learning of difficult topics in chemistry. (Contains 4 tables.)