Speech communication often takes place in noisy environments, including situations where two or more voices compete for the attention of the listener. Experiments with simultaneous vowel pairs investigated aspects of a priming stimulus that draws attention to a target voice in a mixture of two voices. Previous studies have shown identification accuracy is higher in conditions where the vowel pair was preceded by a precursor with the same F[inf 0] and spectrum envelope as one of the vowels. These findings are consistent with an explanation based on auditory adaptation. This conclusion is reinforced by results from the present experiments which show that when the precursor is placed after the concurrent vowel pair (a postcursor), there is little benefit to identification accuracy. Thresholds for identifying a target vowel in the presence of a masker vowel when a precursor is present with the same intensity, F[inf 0], and spectrum envelope as the masker vowel suggests that the phenomenon of enhancement appears consistently only when the enhanced vowel and the precursor are at similar intensities. The present experiments suggest that enhancement is the result of perceptual grouping mechanisms that are supplemented by adaptation processes possibly operating at a level more central than the auditory nerve.