Tima has a typologically unusual 12-vowel advanced tongue root (ATR) harmony system, contrasting six [+ATR] vowels /i e ɨ ʌ o u/ with six [−ATR] vowels /ɪ ɛ ɘ a ɔ ʊ/. This harmony system provides a test case for generalisations about ATR systems: for example, that [−ATR] is less compatible with higher vowels; that [+ATR] is less compatible with lower vowels and that central vowels are incompatible with [ATR] systems. After showing that all vowels participate fully in ATR harmony, this article presents an acoustic study of the Tima ATR contrast. We show that /ʌ/, the [+ATR] counterpart of /a/, patterns as a mid vowel, and that duration and voice quality differences characterise Tima’s crowded vowel inventory. Though F1 is the primary individual correlate of the ATR contrast, as is true cross-linguistically, a number of measures support voice quality differences as well, as predicted by the Laryngeal Articulator Model account of ATR systems.