Radiative transfer modeling is used to estimate the effects of nonideal receiver frequency passband characteristics on the measured brightness temperatures from imaging microwave radiometers over the ocean. The analysis includes microwave frequencies from 6 to 40 GHz and applies to the lower frequency channels of conically scanning, space-based radiometers such as AMSR-E, SSMI, SSMIS, and WindSat. The analysis demonstrates that frequency passband characteristics can have significant effects on the brightness temperatures for microwave imaging channels. The largest effects are due to shifts in the center frequency of the passband. The imaging channels near the water vapor resonance at 22.235 GHz are most sensitive to passband characteristics. The effects for these channels depend on the water vapor in the scene.