The goal of this dissertation is to increase our understanding of language variation and change by examining a particular case of linguistic variation in Rio de Janeiro. Diphthongization of back vowels before word final /s/ in words like mas "but" is a commonly noted feature of the Portuguese spoken in Rio de Janeiro. This leads to a potential merger of /a/ and /ai/ in this environment, such that mas is homophonous with mais "more." Diphthongization is examined as a conditioned sound change, and it is shown that phonetic environments that favor large formant transitions also tend to favor diphthongization, both historically and in synchronic variation. Although both /a/ and /ai/ are fully diphthongal before word final /s/ for nearly all speakers in Rio, some differentiate them by fronting and/or raising the onset of /a/. This fronting and raising appears to be a change in progress for the word mais, with younger speakers in the working and middle classes having the most advanced tokens on average. In addition, young female speakers appear to be leading the fronting and raising of the mas onset. The vowel /a/ was also examined before non word final /s/ and /z/ (eg. passa "pass") for comparison. Although /a/ in this environment is more monophthongal than /a/ and /ai/ in the pre-word final /s/ environment, it does show signs of diphthongization, again with young, working class females leading the change. These correlations are similar to those found for sound changes in many other societies, though this is the first time they have been noted in Brazil. Perceptual data on minimal pairs like mas and mais show a lack of symmetry between production and perception, with some speakers producing a distinction that they do not claim to perceive. At first glance, this suggests a case of a near merger. However, upon closer examination, it appears that lexical diffusion is a better explanation for the patterns found in the data. Specifically, words with a higher frequency of usage tend to show more fronting and raising of the vowel than low frequency words.