This paper describes findings from a study to explore Singapore A-level (Grades 11 and 12, 16-19 yr old) students' understanding of ionisation energy, an abstract and complex topic that is featured in school chemistry courses. Previous research had reported that students in the United Kingdom commonly use alternative notions based on the perceived stability of full shells and the "sharing out" of nuclear force, but that such ideas tend to be applied inconsistently. This paper describes results from the administration of a two-tier multiple-choice instrument, the ionisation energy diagnostic instrument, to find (1) whether A-level students in Singapore have similar ways of thinking about the factors influencing ionisation energy as reported from their A-level counterparts in the UK; and (2) how Singapore A-level students explain the trend of ionisation energy across different elements in Period 3. The results indicate that students in Singapore use the same alternative ideas as those in the UK, and also a related alternative notion. The study also demonstrates considerable inconsistency in the way students responded to related items. The potential significance of the findings to student understanding of complex topics across the sciences is considered.