A study of 333 samples from eight sulphate sections of the Badenian (Middle Miocene) from the marginal part of the Carpathian Foredeep basin and one section from its central part proved that the isotopic compositions of both oxygen and sulphur show a similar trend of evolution throughout the sections. In some cases, in the lower part of gypsum sections a gradual decrease of δ³⁴S and δ¹⁸O is observed. In turn, the upper part of the section shows fluctuations of quite high amplitude, but within a clearly defined interval. The variation of δ¹⁸O values in the whole area is similar to that of δ³⁴S values although in particular facies the variation ranges are different. Clastic gypsum shows the greatest spread of δ³⁴S and δ¹⁸O values. 39 % of all δ³⁴S values and 46.5 % of all δ¹⁸O values are higher than those characteristic for the marine sulphate values, and these higher values are mostly characteristic for clastic gypsum. Selenitic gypsum shows relatively narrow ranges of δ³⁴S and δ¹⁸O values and hence is especially useful for analyses of depositional conditions. The isotopic composition of Badenian sulphates reflects the marine origin of brines. The recorded great spread of δ³⁴S and δ¹⁸O values in the Badenian sulphates is related to the recycling of previously formed evaporites already during gypsum precipitation in the Carpathian Foredeep basin.