The Mycenaean tablet TH Uq 434 provides clear evidence that the preposition pa-ro can govern nouns marked with instrumental morphology (specifically instr. pl. in -pi). Elsewhere we also have evidence for unambiguous dative-locative morphology after pa-ro. Although there appears to be a correspondence between morphology and sense (ablatival sense with instrumental morphology, locative sense with dative-locative morphology) which could be used to suggest a syncretism of ablative and instrumental, it is argued that this is illusory, and that the only scenario which satisfactorily explains the data is (i) a dative-locative-instrumental syncretism and (ii) a simplification of the system of case government by prepositions.