Three experiments with 84 undergraduates investigated the effects of expectancy and stimulus frequency in character classification and identification tasks. Both expectancy and frequency had effects on overall response latency, but the 2 factors had different patterns of interaction with 2 other experimental variables. The effect of visual quality was larger for low-frequency than for high-frequency stimuli, whereas the effect of quality was independent of expectancy. The effect of stimulus-response (S) compatibility was more strongly influenced by whether a stimulus was expected than by the frequency of the stimulus. Results suggest that expectancy and frequency have somewhat different effects on human information-processing mechanisms. Expectancy allows temporary preparation for a specific S pair in response selection processes, but it has no effect on perceptual processes. Stimulus frequency, on the other hand, influences the efficiency of both perceptual and response selection processes. The overall effect of stimulus probability may be interpreted as having both expectancy and frequency effects as components.