This paper presents the first sociophonetic study of Cantonese vowels using sociolinguistic interview data from the Heritage Language Variation and Change in Toronto Corpus. It focuses on four allophones [iː], [ɪk/ɪŋ], [uː], and [ʊk/ʊŋ] of two contrastive vowels /iː/ and /uː/ across two generations of speakers. The F1 and F2 of 30 vowel tokens were analyzed for these four allophones from each of 20 speakers (N = 600 vowel tokens). Results show inter-generational maintenance of allophonic conditioning for /iː/ and /uː/ as well as an interaction between generation and sex such that second-generation female speakers have the most retracted variants of [ɪk/ɪŋ] and the most fronted variants of [iː]. This paper will discuss three possible explanations based on internal motivation, phonetic assimilation, and phonological influence. This will illustrate the importance of multiple comparisons (including inter-generational, cross-linguistic, and cross-community) in the relatively new field of heritage language phonology research.