|Abstract:||In Spanish denominal derivation, the stem formative of the base typically disappears before the derivational suffix: e.g. man-o ‘hand’, man-az-a ‘hand.AUG’, *man-o-az-a. This pattern can be analysed in two ways: as driven by a morphotactic restriction, or as created by a morphologically sensitive phonological process of stem-final vowel deletion. Following James Harris, most generative linguists have consistently assumed the former; Stratal Optimality Theory, however, requires the latter, for otherwise the interaction between diphthongization and depalatalization gives rise to a stratification paradox. This paper adduces independent morphological evidence to confirm the existence of stem-final vowel deletion in Spanish, as predicted by Stratal Optimality Theory. Our data reveal a previously unrecognized contrast between pseudoplural nouns and nouns with athematic stems ending in /s/, and cast light on the relative rôles of suffixes and infixes in diminutive formation.