It is argued that a model of goal-independent spreading activation in a social or semantic knowledge structure is insufficient to explain implicit association effects in the IAT (Greenwald, McGhee, & Schwartz, 1998). An alternative account is proposed, which attributes IAT effects to differential costs for switching between task sets. Two experiments were conduced to test this account. In Experiment 1, specific task-set switching cost was a function of IAT condition: switching between tasks was associated with significantly more cost in the incompatible IAT phase. In a second experiment the magnitude of the IAT effect was reduced when task-set reconfiguration was possible in advance of or simultaneously with the upcoming stimulus. The results are discussed with respect to recently suggested accounts of the effect.