A growing body of neuroimaging data suggests that direct measurements of brain activity can reveal subliminal effects that remain invisible with behavior measures alone. We examined whether sentence comprehension processes could be triggered by a sequence of masked words. On each trial, participants viewed a rapid sequence of masked or unmasked words, including a subject noun, three adverbs and followed by a visible target verb. To probe the capacity limits of unconscious processing, we measured event-related potentials associated with the semantic congruency between the noun and the verb, while varying the subject position in each sentence. Unmasked sentences produced significant behavioral effects of congruency, paralleled by robust N400 effects, independently of subject-verb distance. By contrast, masked sentences produced no behavioral effect and elicited N400 effects only when subjects and verbs were separated by 0 or 1 word. The present results suggest that semantic integration of multiple words can occur unconsciously only if the distance between the words to be integrated does not exceed two words. Although the possibility remains that even longer sequence of invisible words may produce similar neural effects in different experimental settings, our ERP data show that only conscious perception gives access to a buffer that enables robust sentence-level processing independently of temporal distance.