Functional measurement was utilized to study the acoustic cues that contribute to the perception of the voicing difference in /zi/ and /si/. Rather than simply varying the acoustic signal along a single dimension and observing the effect on perception, changes along two acoustic dimensions were covaried in a factorial manner. The time between the onset of the syllable and the onset of vocal‐cord vibration called voice onset time (VOT) was covaried with the fundamental frequency (F0). Observers were asked to indicate where each stimulus fell on a scale from /zi/ to /si/. The results showed that both VOT and F0contribute to the perception of voicing. Sounds were judged as more /zi/‐like with decreases in VOT and with decreases in the F0. The frequency contour of F0during the syllable had very little effect beyond that accounted for by the frequency of F0at the onset of vocal‐cord vibration. Another experiment showed that the role of F0could not be attributed to the possibility that there was less energy at the first formant with higher frequency values of F0. A quantitative model assuming that VOT and F0are perceived independently provided a good description of the observed judgments.Subject Classification: [43]70.30, [43]70.20, [43]70.70.