In this study, some aspects of single-channel compression have been investigated in 15 hearing-impaired subjects and 4 normal-hearing listeners. Phoneme perception as a function of the compression-threshold level was measured for two hearing aids with input-dependent compression, differing in compression ratio and compression recovery time. Phoneme-identification scores, obtained by different hearing aids under similar conditions, did not differ significantly. However, there was a small but significant increase in identification scores for the lowest setting of the compression threshold. Some qualitative differences in the patterns of confusion could also be established. There are indications that compression reduces the relative contribution of frication and enhances the contribution of the second formant of vowels in phoneme perception.