This study aimed to describe seventh grade English and Turkish students' levels of understanding of decomposition. Data were analyzed descriptively from the students' written responses to four diagnostic questions about decomposition. Results revealed that the English students had considerably higher sound understanding and lower no understanding than the Turkish students while the Turkish students' level of misunderstanding of decomposition was lower than those of the English students. The English and Turkish students' responses tended to show similar partial understanding in explaining decomposition. Difference between levels of understandings was significant according to result of chi-square test. (Contains 5 tables and 2 figures.)