In this paper we investigate the role of semantic relations in grammatical alternations. The specific alternation we look at is that between the proper name modifier construction, e.g. the Obama government, and the determiner genitive, e.g. Obama’s government. Through the use of an experimental study in which participants were asked to rate the naturalness of the two constructions in 20 attested natural language contexts and provide paraphrases of the semantic relations in question, we tested when the two constructions alternate and whether either construction expresses semantic relations that block alternation. Our initial finding is that none of the relations we studied is categorically associated with only one of the constructions, but that certain relations – notably possession and name – are far more preferentially associated with determiner genitives and proper name modifiers respectively. Despite these “default” associations, participants nevertheless identified a range of possible interpretations for many of the examples, meaning that our study simultaneously supports the opposing theoretical views of default relations and semantic underspecification. Further, our study validates the inclusion of semantic relations in genitive alternation studies as a major factor despite the notorious difficulties in their operationalisation. Animacy distinctions, although more straightforward to codify, appear to be of lesser importance. Methodologically, our study shows the value of an experimental approach as a corrective to researcher intuitions about the identification of semantic relations in context.