The study is concerned with the mechanisms by which pre-and post-aspiration are related synchronically and diachronically in Andalusian Spanish. An analysis of isolated word productions by 24 speakers of a Western Andalusian and by 24 of an Eastern Andalusian variety each divided into two age groups provided evidence for a sound change in progress: younger speakers and those of the Western Andalusian variety were more likely to produce /st/ with a shorter pre-aspiration and with a longer post-aspiration phase than their older and Eastern Andalusian counterparts. The results of further between-group duration analyses show that the sound change cannot be entirely explained by a realignment from an anti-phase to an in-phase timing relationship between the closure and glottal opening; they also provide evidence for the development of a trading relationship between the duration of the closure and the duration of post-aspiration. In order to test whether perceptual factors might have contributed to this sound change, a perception experiment was carried out. Listeners of Argentinian Spanish judged stimuli of a minimal pair pasta-pata with variable pre-aspiration duration in a forced-choice perception test. When the continuum was synthesized with a slightly post-aspirated stop release, listeners were more inclined to perceive pasta, which suggests that post-aspiration is parsed with pre-aspiration and serves as a cue for the underlying /st/ even in a non-post-aspirating variety of Spanish.