We tested 3 mechanisms of involuntary attention: (1) a perceptual enhancement mechanism, (2) a response-decision mechanism, and (3) a serial-search mechanism. Experiment 1 used a response deadline technique to compare the perceptual enhancement and the decision mechanisms and found evidence consistent with the decision mechanism. Experiment 2 used a multiple-targets paradigm to compare the decision and serial-search mechanisms. The results favored the decision mechanism. Experiment 3, which varied the display size and whether distractors were present in the display, found that when locating the target was easy, the results conformed to the decision mechanism. However, when locating the target was difficult, the serial-search mechanism was favored. Thus, there appears to be at least 2 mechanisms of involuntary attention. The serial-search mechanism accounts for involuntary attention when the target is difficult to locate, whereas the decision mechanism accounts for results when the target is easy to locate. (Contains 1 table, 7 figures and 6 footnotes.)