The effects of combining two equally effective maskers were studied in normally hearing and elderly hearing-impaired subjects. The additivity of nonsimultaneous masking was investigated by measuring thresholds for a brief 4-kHz signal in the presence of a broadband-noise forward masker, a backward masker, and a combination of both. For the normally hearing subjects, combining two equally effective nonsimultaneous maskers resulted in up to a 15-dB greater increase in threshold than the 3 dB predicted by an energy-summation model (“excess masking”). However, the hearing-impaired subjects showed little or no excess masking. The difference between the two groups is consistent with a theory linking excess masking to the compressive transfer function measured on the basilar membrane (BM). In the hearing-impaired subjects the transfer function is more linear, accounting for the lack of excess masking. The additivity of simultaneous masking was investigated by measuring thresholds for a 100-ms 4-kHz signal in the presence of either a 400-ms broadband noise masker or a 400-ms sinusoidal masker at the same frequency as the signal, and then combining two equally effective maskers, a noise and a tone. The maximum amount of excess masking (3 to 4 dB) was similar across the two groups of subjects, consistent with an explanation based on the use of different detection cues for the tonal and noise maskers. It is argued that, while peripheral compression may underlie excess masking for pairs of nonsimultaneous maskers, it is unlikely that in simultaneous masking, where the maskers are close in frequency to the signal, the two maskers are compressed individually before their effects are combined. It is further suggested that BM nonlinearity may underlie the effects of the upward spread of masking and the nonlinear growth of forward masking, as well as accounting for the additivity of simultaneous masking when the masker frequencies are well below that of the signal.