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Description: Semantic knowledge includes taxonomic and thematic relationships, which may comprise complementary semantic sub-systems, or a single semantic system with differing control processes. The latter view is most clearly supported by a recent report (Thompson et al., 2017) suggesting that low-strength thematic relationships (i.e., relations based on participation in the same event or scenarios) require more effort or cognitive control than low-strength taxonomic relationships (i.e., relations based on similar internal features). If retrieval of low-strength thematic relations is particularly effortful, then it should also be associated with a hallmark physiological measure of cognitive effort: pupil dilation. We tested this hypothesis in a preregistered study using a semantic relatedness judgement task that manipulated semantic type (thematic vs. taxonomic) and relatedness strength (high vs. low) of word pairs. Cognitive control in the similarity task was examined using task-evoked pupillary response (TEPR), as well as standard behavioral measures (reaction times and accuracy). Compared high-strength relations, low-strength semantic relations elicited larger TERPs, slower reaction times, and lower accuracy, consistent with higher control demands. Compared to thematic relations, taxonomic relations also elicited larger TERPs, slower reaction times, and lower accuracy, suggesting that retrieving taxonomic relations required more cognitive effort . Critically, our pupillometric data indicated that controlled processing was particularly important for low-strength taxonomic pairs rather than low-strength thematic pairs. These findings have implications for current models of taxonomic and thematic processing.