Changes in the identification of speech sounds following selective adaptation are usually attributed to a reduction in sensitivity of auditory feature detectors. An alternative explanation of these effects is based on the notion of response contrast. In several experiments, subjects identified the initial segment of synthetic consonant-vowel syllables as either the voiced stop [b] or the voiceless stop [ph]. Each test syllable had a value of voice onset time (VOT) that placed it near the English voiced-voiceless boundary. When the test syllables were preceded by a single clear [b] (VOT = -100 msec), subjects tended to identify them as [ph], whereas when they were preceded by an unambiguous [ph] (VOT = 100 msec), the syllables were predominantly labeled [b]. This contrast effect occurred even when the contextual stimuli were velar and the test stimuli were bilabial, which suggests a featural rather than a phonemic basis for the effect. To discount the possibility that these might be instances of single-trial sensory adaptation, we conducted a similar experiment in which the contextual stimuli followed the test items. Reliable contrast effects were still obtained. In view of these results, it appears likely that response contrast accounts for at least some component of the adaptation effects reported in the literature.