This study examines the discrimination of words ending with voiceless stops /p t k/ in first language (L1) and second language (L2) by three groups of native Thai participants. These participants differed in their countries of residence and experience with L2 English in the formal education system. The first group (T1) was a group of 18 Thai listeners who were living in Australia. The second and third groups consisted of 12 university students (T2) and 12 high-school students (T3) living in Thailand. Eighteen Australian English (AusE) listeners were included as controls. English and Thai words minimally contrasting in the final stop (e.g. 'cap' vs. 'cat') were presented to the listeners to investigate whether L2 phonetic learning occurs even for the contrasts that are expected to be discriminated with high accuracy from the onset of L2 learning and if so, how it is influenced by L2 experience. All three Thai groups showed reasonably accurate discrimination for both English and Thai words, but only T1 showed discrimination accuracy comparable to AusE in English. Further, only T3 was clearly more accurate in discriminating unreleased Thai than English stop contrasts, most of which were accompanied with release bursts. These two findings are taken to be evidence for phonetic learning of specific aspects of L2 contrasts rather than positive L1 transfer.