Abstract In the present study, the ERP (event-related brain potentials) technique was used to investigate how and when prosodic boundaries interact with ongoing discourse context during on-line syntactic processing and especially the precise time characteristics of this prosodic boundaries effect. Chinese question–answer dialogues were used as stimuli. The answers were syntactically ambiguous phrases, the meaning of which could be biased via changing the preceding question context or the prosodic boundaries in the carrier sentence. The results revealed that, first, presence of prosodic boundaries, relative to absence of these boundaries, evoked a P2 effect. Second and importantly, there was an immediate interaction between discourse context and prosodic boundaries. When the prosodic boundaries were inconsistent with the syntactic interpretation built upon the ongoing discourse context, a left-anterior distributed LAN effect or a combined LAN and N400 effect was elicited (time-locked to the critical words at the immediate right side of prosodic boundaries). The results indicated that prosodic boundaries can be used to guide syntactic parsing and can be immediately integrated with the ongoing discourse context during spoken discourse comprehension. In addition, the LAN effect elicited by prosodic boundaries violation indicated that prosodic information may affect the initial incorporation of a word into the syntactic structure in speech processing.