In this study, we determine the acoustic correlates of primary and secondary stress in Tongan. Vowels with primary stress show differences in F0, intensity, duration, F1, and spectral measures compared to unstressed vowels, but a linear discriminant analysis suggests F0 and duration are the best cues for discriminating vowels with primary stress from unstressed vowels. Vowels with secondary stress are mainly marked by differences in F0 relative to unstressed vowels. With regards to the effects of stress on the vowel space, we find that all five Tongan vowels are higher in the vowel space (have lower F1) when unstressed. Moreover, there is no reduction in the overall size of the vowel space. We interpret this pattern as evidence that unstressed vowels in Tongan are not prone to centralization, vowel reduction, or undershoot. The results, however, are consistent with a sonority expansion account (Beckman et al. 1992), whereby stressed vowels are lowered to enhance sonority.