During reading, the model of the situation described by the text is continuously accommodated to new text input. The hypothesis was tested that readers are particularly sensitive to diagnostic text information that can be used to constrain their existing situation model. In 3 experiments, adult participants read narratives about social situations that were ambiguous in terms of whether they involved 2 or 3 social cliques. Diagnostic text information enabling the reader to constrain the situation model to 1 of the 2 potential versions required longer reading times than non-diagnostic information, and fundamentally affected the structure of the situation model. Recognizing the diagnostic value of critical text information did not require additional working memory resources, but updating the situation model according to the diagnostic information did. This evidence may suggest that some of the situation model updating is occurring offline. The results are discussed with regard to working memory resources required in updating situation models. (Contains 4 tables, 2 figures, and 1 footnote.)