Based on a task-set switching account of the Implicit Association Test (IAT), the authors predict a specific pattern of aftereffects as a consequence of working through IAT blocks. In Study 1, performance in an evaluative decision task, but not in a color-naming task, was decreased after working through the incompatible rather than compatible block of a flower-insect IAT. In Study 2, response latencies in an evaluative rating task, but not in a color-rating task, were analogously affected, whereas the ratings themselves were not a function of the compatibility of prior IAT blocks. The aftereffects demonstrate reactivity of the IAT; they bear on the mechanisms underlying the IAT and on compatibility-order effects.