This paper is an acoustic study describing the vowel spaces of bilingual Quechua–Spanish speakers in Cochabamba, Bolivia, with a particular focus on the height of mid and high vowels, in order to explore longstanding questions about the nature of Quechua vowel categories. South Bolivian Quechua is generally described as phonemically trivocalic with high-vowel lowering in presence of uvular consonants, but there has been little acoustic study of vowel formant characteristics (Cerrón-Palomino 1987, Laime Ajacopa, Cazazola & Pairumani 2007). The current study quantifies previous impressionistic observations of the vowel system and describes the acoustic properties of the Quechua vowels. Eleven bilingual speakers of South Bolivian Quechua and Spanish were recorded in a Quechua translation task and a Spanish list-reading task. Results of Bayesian models indicate that Quechua high vowels do appear to undergo systematic lowering following uvular consonants, supporting earlier descriptions. The results also demonstrate that lowering is consistent throughout the duration of the vowels, challenging a purely coarticulatory motivation for the observed lowering pattern. The allophonic mid-vowels of Quechua and the phonemic high and mid vowels of Spanish also have formant characteristics that may differ from one another, providing evidence for the hypothesis that these bilingual speakers could potentially command two separate vowel systems with different bases of articulation. These findings are also relevant for questions related to the nature of three-vowel systems, and whether or not they may contain true mid-vowel allophones.