Two genetically different groups of Hercynian granitoids occur in the Slavonian Mountains which are included in the southern Pannonian Basin. I-type granitoids occur in Barrovian-type progressive metamorphic sequences which originated during the Hercynian orogeny from the Late Silurian to Lower Carboniferous magmatic-sedimentary complex. S-type granitoids, enriched in incompatible trace elements, are accompanied by penecontemporaneous migmatites which originated from rocks of the same progressive metamorphic sequences and lower continental crust. I-type granitoids are represented mostly by granodiorite and monzogranite impoverished in incompatible trace elements, with rare diorite and monzodiorite and basic to intermediate rocks. Hercynian age of the crystalline rocks is supported by numerous K-Ar, 40Ar-39Ar and Rb-Sr measurements carried out mostly on monomineralic concentrates. About 20 representative samples of S-type and I-type granites and associated rocks were selected from over 1000 samples and analyzed in detail for major and trace elements, including REE, Sr and O isotopic compositions; microprobe chemical composition of the main rock-forming minerals was determined. Although most major and trace element diagrams do not provide the best genetic discrimination between the Slavonian granitoids, Sr and O isotope composition, REE data and some other data for the S-type granitoids are indicative of their sedimentary and continental crust source, whereas the I-type granitoids were derived by partial melting of the upper mantle with slight crustal contamination.