This research study was aimed at an investigation of Thai EFL learners’ collocational competence, with focus on the problems in their collocation use. The data, elicited from essays written by two groups of participants with different L2 proficiency levels, are indicative of actual problems with which the learners are really confronted. With respect to the sources of these collocational errors, native language transfer seems to be the most important contributing factor. Where the collocations in L1 Thai and L2 English are incongruent, deviations often arise. The interlingual errors found pertain to preposition addition, preposition omission, incorrect word choice, and collocate redundancy. It is worth noticing that the high-proficiency learners heavily depend on collocational patterns from their mother tongue, to which low-proficiency students are expected to resort. In addition to L1 transfer, the participants also seem to rely on synonymy and overgeneralization, both of which result in erroneous collocations in English. Index Terms—collocation, collocational competence, English learners, difficulty and errors, language transfe