The radiation of the gastropod Melanopsis in the Pannonian basin of eastern and central Europe provides an excellent case study of the tempo and mechanisms of evolutionary diversification. We analyzed the carbon and oxygen isotopic ratios of melanopsid shells from before, during, and after the radiation in order to provide a more detailed paleoenvironmental framework in which to interpret the morphological changes observed. Our samples fall into two groups: those from before and during the radiation (late Sarmatian and Pannonian stages) form a tight cluster; a second, more scattered group of points represents samples from after the radiation (Pontian Stage). The late Sarmatian-Pannonian samples have higher ratios of both isotopes (means of -2.11 for δ18O, and 1.27 for δ13C) than do the succeeding Pontian Stage samples (-4.16 and -2.22, respectively). We interpret this shift as indicative of a basinwide drop in salinity, an interpretation supported by paleofaunal evidence. Our isotopic data refine the environmental scenario in two important ways. The tight cluster of late Sarmatian-Pannonian data indicates that the shallow waters of the basin were relatively uniform and relatively stable with respect to salinity, rather than locally variable or steadily changing across this time interval. The shift to fresher water in the Pontian Stage coincides with the extinction of two widespread and abundant melanopsid species, but several species go extinct earlier than the salinity drop, and two species pass through it seemingly unaffected.