Listeners must integrate multiple sources of information to construct an interpretation of a sentence. We concentrate here on the alignment of prosodic and syntactic grouping during online sentence comprehension. We present the results from a pupillometry study on the use of prosodic boundaries in resolving well-known attachment ambiguities. Using growth curve analyses to capture the non-linear dynamics of pupil dilation, we found increased pupil excursions for sentences that were disambiguated towards the dispreferred, non-local relation, especially when accompanied by supporting prosodic information. However, when prosodic and structural information did not align, pupillary response was muted, potentially indicating a failure to commit to a specific interpretation. More generally, the study shows how the currently under-utilized pupillometry method offers insights into spoken language comprehension.