This study investigated acoustic-phonetic characteristics of English vowels (four monophthongs /I A U V/ and two diphthongs /ei οu/) spoken by native speakers of Australian English (AusE, n = 6) and Thai (T, n = 15). The Thai speakers had lived in Australia for an average of 3.2 years. While the Thai speakers did not differ significantly from the AusE speakers in their vowel quality for monophthongs, they were more dissimilar in their production of diphthongs. Specifically, the Thai speakers produced English diphthongs with less formant movement, a phenomenon widely reported for speakers of various other Southeast Asian languages. Differences between the Thai and AusE groups reached statistical significance for the female speakers. As for temporal characteristics, the Thai speakers produced significantly shorter monophthongs, but not diphthongs, than did the AusE speakers. As a result, the ratio of monophthongs to diphthongs was much smaller for the Thai (0.53) than for the AusE group (0.72). It appeared that the Thai speakers equated the English diphthongs /ei/ and /οu/ with the long vowels /e:/ and /o:/, respectively, in their first language (L1) Thai. Further, their production of the diphthongs may be related to a large number of diphthongs in the Thai vowel inventory, which may encourage its speakers to substitute existing L1 categories for the English diphthongs rather than forming authentic, new phonetic categories.