In Montreal French, a process of diphthongization affects long vowels: those that are inherently long due to historical compensatory lengthening (Yaeger-Dror and Kemp, 1992), and those that are allophonically lengthened before voiced fricatives and /R/ (Dumas, 1981; Santerre and Millo, 1978). Our quantitative analysis of diphthongization in real time examines both the trajectory of this change through the community as well as individual speakers’ participation in it across their lifespans. Our study also provides acoustic measurements of the Montreal French vowel system. We tracked individuals’ vowel trajectories across a 24-year span for a panel of six speakers of diverse social classes. Matched trend samples from the 1971 and 1984 Montreal corpora, with four speakers sampled per year, provide a picture of the community as a whole. We find that four vowels show significant lowering and/or backing in the community, and that all long vowels show decreased diphthongization. Some panel speakers’ longitudinal movements mirror these changes, while other speakers are stable across their lifespans and still others show apparently anomalous movements. We discuss these results and their interpretation.