This thesis is concerned with the mechanisms by which pre-aspiration changes into post-aspiration in Andalusian Spanish. In this variety pre-aspiration is the result of the weakening of syllable final /s/ before a voiceless stop (e.g. [ˈpasta] -> [ˈpahta]). An apparent-time study was carried out to investigate whether there is a sound change in progress from pre- to post-aspiration ([ˈpahta] -> [ˈpatha]) in Andalusian Spanish. An acoustic analysis of isolated words (e.g. espanto, estado, escapa) produced by younger and older Eastern and Western Andalusian speakers was carried out. The results provided evidence for a sound change in progress by which pre-aspiration is gradually giving way to post-aspiration. Further durational analyses suggest that the sound change is more advanced in Western than in Eastern Andalusian Spanish, and that post-aspiration lengthening and pre-aspiration shortening are not directly linked. A perception experiment was carried out to test whether listeners of Andalusian Spanish use post-aspiration to distinguish /t/ and /st/ in the minimal pair /pata/-/pasta/. Younger listeners and Western Andalusians, who produced a longer post-aspiration, were also more sensitive to post-aspiration as a cue to /st/ than Eastern Andalusians and older listeners. The aim of a second perception experiment was to test how listeners of Argentinian Spanish, a non-post-aspirating variety, perceive pre- and post-aspirated stops. The results suggest that in a stimulus [ˈpahtha] with pre- and post-aspiration, post-aspiration is perceptually more prominent. These findings support a model of the Andalusian sound change in which not only articulatory but also perceptual factors are involved.