The experiments reported in this article used a delayed same/different sentence matching task with concurrent measurement of eye movements to investigate the nature of the plausibility effect. The results clearly show that plausibility effects are not due to low level lexical associative processes, but arise as a consequence of the processing of the earliest or most basic form of sentential meaning. In fact, when sentential implausibility and lexical association are varied simultaneously, it is only sentential implausibility that exerts an effect. Effects of implausibility occur rapidly--sometimes parafoveally--and are localised in the regions of the sentence where the implausibility occurs, suggesting an incremental interpretive process progressing on a roughly word-by-word basis. It is suggested that plausibility effects result from the operation of a heuristically-driven process of sentential interpretation. This appears to behave in a "modular" fashion, despite being influenced by real world knowledge and probabilities.