This paper presents the results of a study of the acoustic cues associated with the tense/lax distinction in Bai, a Tibeto-Burman register tone language spoken in Yunnan, China. The purpose of the paper is to provide baseline adult data for comparison with infant speech in an acoustic study of infants’ acquisition of Bai register tones in the second and third years of life. The results show that among adults, F0, F1, and spectral tilt combine to create the tense/lax contrast in Bai. While these three cues tend to be correlated, individual speakers differ in their use, particularly spectral tilt. The patterns in this study suggest that as Bai infants acquire tones in the second and third years of life, their utterances are likely to become structured around these three acoustic cues in previously unattested ways that exemplify the complex interaction between universal physiological and developmental tendencies and the ambient phonological tone system of Bai.