The resolution of ambiguous pronouns is influenced by the preceding linguistic discourse. This raises the question whether the processing of an object pronoun is also influenced by the preceding sentential subject. In an experiment with Dutch adults, we recorded pupil dilation as a measure of the cognitive effort involved in resolving pronominal versus full noun phrase (NP) subjects and pronominal versus reflexive objects. Our results indicate that more effort is needed to resolve a pronominal subject or object compared to a less ambiguous full NP subject or reflexive object. These results support the hypothesis that the ambiguity of a referring expression influences processing. Contrary to our expectations, no evidence was found that the form of the subject influences the processing of a subsequent pronominal object. We conclude that pupillary responses reflect ambiguity resolution in pronoun processing, and that the process to resolve pronouns commences as soon as the pronoun is encountered.