It has been intensely debated whether visual stimuli are processed to the point of semantic analysis in the absence of awareness. In the present study, we measured the extent to which the meaning of a stimulus was registered using the N400 component of human event-related potentials (ERPs), a highly sensitive index of the semantic mismatch between a stimulus and the context in which it is presented. Observers judged the semantic relatedness of a context and target word while ERPs were recorded under continuous flash suppression (Experiments 1 and 2) and binocular rivalry (Experiment 3). Finally, we parametrically manipulated the visibility of the target word by increasing the contrast between the target word and the suppressive stimulus presented to the other eye (Experiment 4). We found that the amplitude of the N400 was attenuated with increasing suppression depth and was absent whenever the observers could not discriminate the meaning of suppressed words. We discuss these findings in the context of single-process models of consciousness, which can account for a large body of empirical evidence obtained from visual masking, attentional manipulations, and, now, interocular suppression paradigms.