In this thesis, a new methodology is proposed for investigating Spanish [s] lenition (sound weakening or loss) via morphological analysis instead of phonetics. Word-final [s] is a morphological plural marker in Castilian Spanish, but is rarely produced in Western Andalusian Spanish (WAS). It is often asserted in the literature that the loss of [s] in WAS requires plurality to be expressed via alternative means. The results of this study rule out lexical and morpho-syntactic compensation for [s] lenition in WAS in several previously untested domains, and imply that there is no functional motivation in Modern Spanish driving a need for compensation for word-final [s] lenition on nouns or determiners. This investigation is built on a predictable calculation of the environments in which the loss of [s] may result in derived singular/plural homophony in WAS nouns. This is used to quantify potential semantic ambiguity. A frequency comparison of 27,366 WAS and Castilian nouns, across 60 specific Determiner + Noun phrase environments, finds no significant differences between the dialects in the type or token frequencies of numerically ambiguous nouns, nor in 98.7% of the tested phrase environments. When taken in context with studies excluding phonetic compensation in WAS, the current results suggest that the low semantic relevance of word-final [s] in Modern Spanish is a potentially far-reaching explanation for the variable manifestations of [s] lenition experienced in Spanish dialects across the world. v ACKNOWLEDGMENTS Thank you to Derek, Tommy, and Annie for all the love, support, and patience while I've been in school. I am deeply grateful to (and for) all of you. To my Mom, and all my extended family and friends, I could not have done this without you. Thank you for being my village. Thanks to my first Spanish linguistics professor, Dr. Máximo Torreblanca of UC Davis, who sparked my love of Spanish into a broader fascination with linguistic structure, and also to my last Spanish linguistics professor, Dr. Juan Sempere of SJSU, who brought me full-circle back to Spanish phonology twenty-five years later, and helped get this thesis off the ground in 2015. Heartfelt thanks to everyone on the 4 th floor of Clark Hall at SJSU, for the perpetually open doors, the willingness to help, or sometimes just to listen, and for being such outstanding human beings. Particular thanks to Drs.