A diagnostic test with three parts was administered to 169 Ss: 40 Thai, 48 Japanese, 38 Spanish, and 43 Other. The analysis showed no significant difference in the groups on total scores; all four groups had difficulty with English prepositions. There were no items that were specifically Thai problems. Rather, their problems with prepositions were shared by all groups. A contrastive analysis via a “translation” task provided information on how English and Thai prepositions differ. The results showed that while English and Thai preposition systems contrast in many ways and predictions can be made based on these contrasts, error analysis was just as effective in showing problems Thai students have with English prepositions.