This paper presents the results of an acoustic analysis of English diphthongs produced by three L1 speakers of Hindi and four L1 speakers of Punjabi. Formant trajectories of rising and falling diphthongs (i.e., vowels where there is a clear rising or falling trajectory through the F1/F2 vowel space) were analysed in a corpus of citation-form words. In line with previous research, the diphthong inventory included six different diphthongs and a long monophthongal vowel [o (length mark)] in place of /[schwa][upsilon]/ in GOAT; however, none of the speakers produced a full set of diphthong vowels. In addition, the /ei/ diphthong, as in FACE, and the /[upsilon][schwa]/ diphthong, as in TOUR, had both monophthongal and diphthongal realizations depending on the speaker. Overall, there was a great deal of variation in diphthong realization across the corpus but L1 appeared to be a relevant factor. Punjabi speakers showed a wider range of phonetic realizations for some of the vowels, and were more likely to produce long monophthongs rather than diphthongs. The results also highlight differences in the phonetic characteristics of several diphthongs between the speakers of two language backgrounds. The results of this study therefore contribute to the debate on the phonemic representation of IE vowels by taking into account different L1 influence (i.e., Hindi or Punjabi).