This paper aims at presenting a quantitative study of diphthongizationresulting from anterior glide insertion in final stressed syllables closed by /S/, suchas nós - nois, mês - meis, in Portuguese as spoken in Porto Alegre, Florianópolisand Curitiba. The investigation follows the theoretical assumptions of variationstudies Figures related to linguistic features have shown that low vowels followedby alveolar fricative favor rule application when this segment belongs to the rootof the word and joins the following word. Extralinguistic features have revealedthe rule is strongly related to geographic variation. A curvilinear pattern has alsobeen observed, suggesting linguistic change in progress.