We report the results of a pupillometry experiment on the prosodic disambiguation of PP modifier sentences. We manipulated instrument plausibility (‘pen’ vs. ‘gun’) and prosodic boundary placement, so that a prosodic boundary biased towards a Modifier [A] or Instrument [B] construal of the prepositional phrase (‘The artist sketched [A] the man [B] with the pen/gun’). A growth curve analysis of change in pupil size suggests that listeners attend to prosodic boundary locations that disambiguate the structure, and that an online processing penalty, as indexed by increased pupil dilation, is generated when prosodic boundary information leads to an implausible construal.