Accumulating evidence indicates that co-occurring alexithymia underlies several facets of the social-emotional difficulties common in individuals with autism spectrum disorder. The mechanisms involved, however, remain poorly understood because measuring alexithymia relies heavily on self-report. To address this issue, carefully matched groups of individuals with autism spectrum disorder and comparison participants rated 70 emotion-inducing pictures on subjectively experienced arousal while skin conductance responses were monitored objectively. The results demonstrated reliable correlations between these subjective and objective measures, and in both groups, around 25% of individual differences in this correlation (i.e. in emotion-relevant interoception) were accounted for by self-reported alexithymia. In the context of the wider literature, this suggests that alexithymia involves a disruption in how physiological arousal modulates the subjective experience of feelings in those with and without a diagnosis of autism spectrum disorder. Since mindfulness-based therapies foster greater awareness of thoughts, feelings and bodily sensations, the findings also have implications for how the symptoms and consequences of alexithymia (e.g. anxiety) might be ameliorated.