This paper examines the phonetic interactions of tone and voice qualities in Sylheti. Data from six native speakers are examined to understand the voice qualities of the vowels carrying contrastive tones. The results identify three spectral measures (viz., H1*–H2*, H1*–A2*, and H1*–A3*) and one noise measure (viz., CPP) as reliable indicators of modal (or in the continuum of modal to tense) vs. breathy (or, in the continuum of breathy to lax) phonation contrasts in the vowels carrying high and low tone, respectively. Finally, a statistical model is proposed that predicts consistent phonation contrasts across the total duration of the contrastive tones.