Frequency effects in auditory backward masking were examined by psychophysically determining the thresholds of 0.5, 1, 3, 5, and 7 kHz tone burst probes followed at different delay times by a noise burst masker. It was found that for each probe frequency condition, the amount of backward masking (defined as the shift in probe threshold due to the presence of the masker) as a function of the time interval between the probe and masker (Δt) could be approximated reasonably well by a simple decaying exponential function. The rate of decay of the masking effect as a function of Δt was found to increase with increasing probe frequency. This latter result qualitatively supports Duifhuis's [J. Acoust. Soc. Am. 54, 1471–1488 (1973)] theory which attributes short term backward masking to the temporal overlap of cochlear responses to the probe and masker.