The Simon effect was studied using ERPs and the lateralized readiness potential (LRP). In Experiment 1, visual stimuli requiring left or right hand responses were presented at either side of fixation. In Experiment 2, the empty side was occupied by a distractor. Spatial S-R incompatibility delayed P300 latency in both experiments, as predicted by perceptual interference accounts of the Simon effect. However, the LRP results were not conclusive. The lateralized presentation of visual stimuli (Experiment 1) or targets (Experiment 2) produced a posterior ERP hemispheric asymmetry that spread to central electrodes and obscured the early development of the LRP. Experiment 3 avoided the problem by presenting the stimuli in the vertical meridian, and the LRP showed signs of incorrect response preparation in incompatible trials, in agreement with response-interference models. Both perceptual and response interference may be present in the Simon effect.