Appropriate transformations based on the theory of temporal summation were used to determine backward masking as a function of the time interval between the onset of 800‐, 1000‐, 1200‐, and 1750‐cps tones and the onset of an 80‐dB narrow band noise centered at 1000 cps that partially masked the tones. Backward masking was found to consist of at least two phases separated by a transition region in which threshold did not appear to be determined by the duration of the probe tone. Masking at 800, 1200, and 1750 cps was found similar to that at 1000 cps, but did not extend as far back in time. The results are interpreted to mean that masking first occurs for those tones within a noise band and then spreads to those frequencies outside the band until the pattern commonly observed under simultaneous masking is obtained.