This paper presents new experimental data on Quito Spanish /s/-voicing, which has attracted considerable interest from theoretical phonologists owing to the overapplication of voicing to word-final pre-vocalic /s/. Bermúdez-Otero (2011) singles out Quito /s/-voicing as an important test case for discriminating between two competing theories of phonology-morphosyntax interactions: Output-output correspondence and cyclicity. Overapplication in /s/-voicing cannot be captured using correspondence relationship to a base form, which challenges Output-output correspondence as a theory of opacity. However, the argument only holds insofar as word-final pre-vocalic /s/-voicing is considered phonological, as Output-output correspondence can account for /s/-voicing assuming that it only applies in the phonetics (Colina 2009). We discuss the diverging empirical predictions concerning categoricity and gradience in the surface realisation of voicing processes. We further test these predictions based on acoustic data from seven speakers of Quito Spanish. Evidence from speech rate manipulations shows that some speakers produce more voicing during frication at normal speech rate, compared to fast, maintaining a stable voicing ratio across different speech rates. We argue that for these speakers, /s/-voicing is optional but categorical, and so it ought to be analysed as phonological. This result presents a challenge to the Output-output correspondence approach, but can be accommodated within cyclicity.