Previous studies have indicated that A-level students in the UK and Singapore have difficulty learning the topic of ionisation energy. A two-tier multiple-choice instrument developed in Singapore in an earlier study, the Ionisation Energy Diagnostic Instrument, was administered to A-level students in the UK, advanced placement high school students in the USA, and first-year university students in China, New Zealand, and Spain to determine whether the students from different countries and educational systems had similar conceptions and difficulties as the students in Singapore with the concepts assessed in the instrument. The results showed that, in general, the students in all six samples had similar alternative conceptions, which were grouped under the categories of "octet rule framework, stable fully-filled and half-filled subshell conceptions, and conservation of force thinking". The students also resorted to "relation-based thinking" when answering items involving the trend of ionisation energies across Period 3. Implications for teaching and further research are discussed. (Contains 3 tables and 2 appendices: (1) "Examples of items from the Ionisation Energy Diagnostic Instrument"; and (2) "Assessment objectives on ionisation energy in the A-level syllabus for 2001.")