Abstract Convergence in case mapping strategies in the oral speech of Quechua–Spanish bilinguals is examined. In principle, Quechua case suffixes could be analyzed as postpositions or as case markers. The data show that in code mixing utterances, Spanish prepositions and Quechua case suffixes co-occur and that mapping strategies converge. The fact that, in Quechua–Spanish code switching, Spanish prepositions are combined with Quechua case suffixes is taken as evidence that the latter are case markers, not postpositions. Further support for this view is the fact that no obvious differences were found in the data between non-oblique and oblique case markers. This distribution is consistent with an analysis of Quechua case suffixes as the spell out of an agree relationship between a null prepositional head and a DP. Cases of case suffix dropping in differential object marking expressions are analyzed as a possible source of instability in case marking in code mixing utterances.