This paper explores the role of the phonetic context (following consonant, preceding vowel, position in the word and stress) in the voicing assimilation process of coda /s/ in Basque Spanish using an information gap task created by Schmidt and Willis (2011) and adapted to the dialect. The results of the analysis show that the average assimilation rate is 58% and that the degree of voicing is lower than that found by Schmidt and Willis (2011) for Mexican Spanish. Differences were also found between males and females, as well as depending on the manner of articulation of the following consonant and stress (these last two were found to be statistically significant). A correlation was also found between the duration of the segment and the degree of voicing. The study shows that the degree of voicing is influenced by the phonetic context and suggests dialectal differences for this process.