Effects of semantic transparency, reflected in processing differences between semantically transparent ("teabag") and opaque ("ladybird") compounds, have received considerable attention in the investigation of the role of constituents in compound processing. However, previous studies have yielded inconsistent results. In the present article, we argue that this is due to semantic transparency's often being conceptualized only as the semantic relatedness between the compound and constituent meanings as separate units. This neglects the fact that compounds are inherently productive constructions. We argue that compound processing is routinely impacted by a compositional process aimed at computing a compositional meaning, which would cause compositional semantic transparency effects to emerge in compound processing. We employ recent developments in compositional distributional semantics to quantify relatedness- as well as composition-based semantic transparency measures and use these to predict lexical decision times in a large-scale data set. We observed semantic transparency effects on compound processing that are not captured in relatedness terms but only by adopting a compositional perspective.