Tuffs have been deposited during the Miocene in the North Croatian Basin which is located in the southern part of the Pannonian Basin System. In this thesis chemical and mineral composition of the tuffs as well as their age and origin were determined. In addition, the fossil content was analyzed in the underlying and overlying beds of the tuffs. Based on chemical composition tuffs have been classified as neutral (tephriphonolite) to acid (riolite). In most tuffs volcanic glass was partially or completely altered to clay minerals (smectite) or in some cases zeolites (heulandite). During the sin-rift phase of basin development volcanic activity was at its peak. According to the results of the 40Ar*/39Ar dating on sanidine, the most intensive period of volcanic activity was during the Karpatian and the Early and Middle Badenian (17 – 14.4 Ma). For the first time, the volcanic activity in the post-rift stage of the development of the basin has been recorded at the end of Sarmatian ~ 12 Ma ago. Based on the content of immobile trace elements the source of the pyroclastic material was placed on an active continental margin, most probably located in the Eastern Carpathians area. The fossil assemblage of foraminifera, ostracods, nannoplankton and palinomorphs has shown that the tuffs older than 15.1 Ma were deposited in freshwater or brackish lake environments, and those dated from 14.8 to 14.4 Ma in marine environments. Older studies place the onset of the marine sedimentation in Karpatian time, while this study suggests it should be moved to the Early/Middle Badenian boundary.