This paper describes unstressed vowel reduction in the Spanish of the Southern Peruvian Andes. Based on spectrographic analysis of 16,581 unstressed vowels (1648 reduced) extracted from the conversational speech of 16 informants, it is determined that the primary phonetic effect of "reduction" in this dialect is devoicing rather than centralization. The relationship between devoicing rates and vowel type, word position, consonantal context and speech rate are discussed and compared with descriptions of vowel devoicing in other languages. Findings are interpreted within the framework of Articulatory Phonology.