Mounting evidence shows that frequency of use plays a fundamental role in shaping linguistic structure, including phonological structure (cf. Bybee 2001). Because the study of frequency effects is relatively new, our understanding of how they impact structure continues to be refined. This study explores the effects of several frequency measures on the resolution of hiatus between words in Spanish, and reveals that in addition to the traditional phonological factors, frequency is also involved. Multivariate analyses show that ratio frequency - or the frequency of a two-word string relative to that of one of the words it contains - is a better indicator than straightforward token frequency of the likelihood that the string will be processed as an autonomous unit and undergo concurrent phonological reduction. These findings build on a usage-based model of language, providing important insights into the nature of lexical storage and how this relates to linguistic variation and change.