This book studies vowel system of the Hausa language. It particularly examines the morphophonological process of monophthongization of Hausa diphthongs 'ai' and 'au'. It analyses phonetic environment in which monophthongization of "ai" and 'au' occurs in Hausa and where it does not. The book reveals that monophthongization in Hausa shows clustering properties involving several features of post-lexical rules. It also distinguishes between phonological alternations that are conditioned by morphology [lexical rules] and those that are not.