This paper systematically investigates the phonological process of voicing assimilation of the sibilant in Mexican Spanish whereby /s/ is produced as [z] based on the voicing of the following consonant (for example, /mismo/ [mizmo] 'same'). Twelve college aged (21-29 years) native Mexican speakers attending the Universidad Autónoma Metropolitana-Iztapalapa, Mexico, and recorded in Mexico City provided the data. The instrument for the study was a contextualized picture description task presented in a PowerPoint format, which allowed for the systematic investigation of the contexts of /s/ in intervocalic position and in coda position followed by a voiceless or voiced consonant; these contexts were also examined by phrase position. Findings indicate that /s/ voicing before a voiced consonant is not a categorical process in Mexican Spanish and that factors associated with variability in /s/ voicing include the sex of the speaker and phrase position. The paper also documents the consistent, albeit small in duration, manifestation of progressive voicing from the preceding vowel into the sibilant, not previously documented for Spanish.