The diffusion model developed by R. Ratcliff (1978, 1981, 1985, 1988) for 2-choice decisions was applied to data from 2 letter identification experiments. In the experiments, stimulus letters were displayed and then masked, and the stimulus onset asynchrony between letter and mask was manipulated to vary accuracy from near chance to near ceiling. A standard reaction time procedure was used in one experiment and a deadline procedure in the other. Two hypotheses about the effect of masking on the information provided to the decision process were contrasted: (a) The output of perception to the decision process varies with time, so that the information used by the decision process rises and falls, reflecting the stimulus onset and mask onset. (b) The output of perception to the decision is constant over time, reflecting information integrated over the time between the stimulus and mask onsets. The data were well fit by the diffusion model only with the assumption of constant information over time.