This paper presents an acoustic study of the vowel system of Southern Ute, a Southern Numic Uto-Aztecan language spoken in southwestern Colorado. Previous auditory accounts proposed an inventory of five vowel phonemes that participate in three allophonic processes and contrast in length and stress. We investigate how the vowels are realized at the phonetic level by analyzing F1, F2, duration, spectral emphasis and f0 in over 6000 vowel tokens produced by eight fluent speakers (four female and four male). Our findings provide new phonetic detail for the earlier non-instrumental descriptions of the language, including both expected and previously unreported effects. We confirm the existence of five distinct phonemic categories but show that their prototypical phonetic realizations and positional allophones do not always match the earlier descriptions. We also describe how phonemic length and stress are marked in Southern Ute.