Generally, the Sarmatian age is interpreted as an interval of reduced salinity reflecting a transitional interval from fully marine conditions of the Badenian to the non-marine conditions of the Lake Pannon. The Middle to Late Miocene sedimentary sequence of "St. Margarethen - Zollhaus" in Burgenland (Austria) comprises in its Upper Sarmatian (Upper Serravallian) part a very individual rich, low diversity gastropod fauna (Harzhauser and Kowalke 2002). This was attributed to three successive assemblages which were interpreted to indicate changes of environmental conditions from a limnic-fluvial, to a brackish-intertidal into a marine-intertidal environment. This interpretation deviates already from textbook knowledge in reconstructing shallow marine conditions. Oxygen and carbon isotope data of these gastropod shells were now used to test the results of this paleoecological interpretation. In addition to a representative number of shells of the main members of the three assemblages [Potamides hartbergensis (Hilber), Granulolabium bicinctum (Brocchi), Potamides disjunctus (Sowerby)] some shells of freshwater and terrestrial gastropods were used for stable isotope analyses. For detecting diagenetic alterations, samples were analysed for their shell mineralogy and microstructures with X-ray diffraction analyses and scanning electron microscopy prior to stable isotope analyses. Oxygen and carbon isotope data confirmed the freshwater and marine endmembers of the succession. Shells from those assemblage interpreted as brackish environment on a mudflat, however, showed higher oxygen data than the marine endmember. These heavier oxygen values can only be explained by evaporation which affected small protected areas on the mudflat. The results of the stable isotope study improved the already existing paleoenvironmental informations.