This empirical study investigated durational characteristics of English vowels /i/ as in 'heat' and /I/ as in 'hit' produced by two groups of second language (L2) learners who differ in their first language (L1) backgrounds, i.e. Japanese and Thai. In these languages, vowel length is phonemic. Experiment 1 compared the /i/-/I/ contrast produced in monosyllabic words ending with /p t k/ by Australian English (OZ1, n=6) and Thai speakers (T, n=22). Experiment 2 compared the same vowel contrast in monosyllabic words ending with /t/ and /d/ by another group of Australian English speakers (OZ2, n = 12) and Japanese speakers (J, n =24). While both non-native groups showed a greater durational differentiation of the /i/-/I/ contrast than the OZ groups to which they were compared, J and OZ2 differed to a greater extent when the /i/-/I/ contrast occurred before /t/ than before /d/. This finding highlights the importance of taking language-specific phonetic factors such as allophonic variation into consideration in L2 speech acquisition research.