Background and purpose: This study investigated Indonesian and Japanese senior high-school students' understanding of electrochemistry concepts. Sample: The questionnaire was administered to 244 Indonesian and 189 Japanese public senior high-school students. Design and methods: An 18-item multiple-choice questionnaire relating to five conceptual categories (reactions occurring during electrolysis, differences between electrolytic and voltaic cells, movement of ions in voltaic cells, poles in voltaic cells, voltaic cell reactions) was administered. Results: The findings of this study show that difficulties and alternative conceptions previously reported in the literature are held equally by students from a developing and developed country, Indonesian and Japan respectively. Conclusions: Collectively, the findings suggest that students' understanding of electrochemistry concepts is relatively weak. Students from both samples shared common difficulties and displayed several alternative conceptions dealing with electrolysis, electricity flow, the voltaic cell and the electrode reactions. Not surprisingly, the students displayed limited consistency in understanding of the concepts in the five categories. This study has implications for teaching and learning, particularly in classroom discussions using models and computer animations in order to reinforce understanding at the sub-microscopic level. (Contains 7 tables and 2 figures.)