Past studies of modulation detection interference (MDI) have considered the possibility that the effect may in part relate to auditory grouping of the probe and masker. Alternatively as a cross-channel effect, MDI may be representative of informational masking. The present experiments varied modulator waveshape in attempts to distinguish the effects of auditory grouping and informational masking on MDI. In the first experiment, both the slope and duty cycle of the masker modulator varied across conditions with the task to detect either 4- or 10-Hz sinusoidal probe modulation. The effect of masker-modulator waveshape on the detection of 4-Hz probe modulation was generally consistent with change in the perceptual prominence of the masker. The 10-Hz results showed little effect of these manipulations. The second experiment utilized interrupted modulation patterns to allow for either concurrent or sequential envelope fluctuation of the probe and masker. Results showed no effect of modulation concurrency. In both experiments, significant departures from energetic masking in the modulation domain were obtained. Collectively, these results were taken to support an interpretation of the interference in terms of informational masking rather than grouping based on common modulation. [Work supported by NIDCD.]