This work contains a comprehensive description of Kwaza, which is an endangered and unclassified indigenous language of Southern Rondônia, Brazil. The Kwaza language, also known in the literature as Koaiá, is spoken by around 25 people today. Until recently, our knowledge of Kwaza was based on only three short word lists, from 1938, 1943 and 1984. Like the language, the culture and the history of its speakers are undocumented. The Kwaza people as an ethnic group have been decimated by increasing ecological, physical, social and cultural pressure from Western civilisation since contact in the past century. This is the situation for many indigenous peoples of Rondônia and of the Amazon region in general. Linguists expect that the majority of these peoples will cease to exist as distinct language communities during the coming decades. The present work is intended as a contribution to the documentation and preservation of the languages of the Amazon basin. In this respect, Kwaza has represents an especially urgent case in view of its undetermined classification, the lack of documentation and its endangered status. This work is based on the author ́s personal fieldwork conducted between 1995 and 2002, and it consists of three parts. Part I contains a thorough description of the phonology and morphosyntax of the language and a concise overview of its social, cultural and historical context. Part II contains a diverse selection of transcribed and translated texts with interlinear morphological analyses. Part III is a dictionary of Kwaza, including many examples and an English-Kwaza register. This complete description is of interest to linguists in general, scholars of South American languages in particular, and anthropologists and historians interested in the Guaporé region.