This study investigated the processes reflected in the widely observed N400 and P600 event-related potential (ERP) effects and tested the hypothesis that the N400 and P600 effects are functionally linked in a tradeoff relationship, constrained in part by individual differences in cognitive ability. Sixty participants read sentences, and ERP effects of semantic anomaly, relative to plausible words, were calculated for each participant. Results suggested qualitatively different ERP patterns across participants: Some individuals generated N400-dominated effects, whereas others generated P600-dominated effects, for the same stimuli. To specify the sources of individual differences in brain responses, we also derived aggregate scores for verbal working memory (WM), nonverbal WM, and language experience/knowledge, based on 6 behavioral measures administered to each participant. Multiple regression analysis pitting these 3 constructs against each other showed that a larger verbal WM capacity was significantly associated with larger P600 and smaller N400 effect amplitudes across individuals, whereas the other constructs did not predict the ERP effects. The results suggest that N400 and P600 brain responses, which may be attributable to semantic integration difficulty and structural processing, respectively, vie for expression when comprehenders encounter semantically unexpected words and that which option wins out is constrained in part by each comprehender's verbal WM capacity.