Miocene pelitic sedimentary rocks from six wells in the Sava Depression (sub-basin in the south-western part of the Pannonian Basin System) were investigated in order to determine the degrees of diagenetic alteration. Qualitative and semiquantitative mineral compositions of samples and the content of smectite in illite-smectite (I-S) were determined by X-ray powder diffraction (XRD). Vitrinite reflectance and thermal alteration index (TAI) were measured in order to estimate the palaeotemperatures. Carbonate minerals, clay minerals and quartz are the main constituents of the pelitic sediments. Feldspars (albite), pyrite, opal-CT and hematite are present as minor constituents in some rocks. The mineral composition of the rocks, apart from previously known differences caused by various depositional environments and clastic material provenance, is dependent on the degree of diagenetic processes. At elevated temperatures and large burial depths the minerals formed by alteration processes or precipitated at the surface; smectite, kaolinite and calcite were gradually replaced by minerals formed by diagenetic processes, i.e. by illite-smectite, illite, chlorite, Ca-excess dolomite/ankerite and albite. Based on XRD data for smectite, I-S and illite, three stages of diagenetic development have been established. The early stage was characteristic of samples at depths <1.8 km containing smectite, I-S of the random R0 type and detrital illite. The middle stage begins with the appearance of the ordered R1 type of I-S at the depths >1.8 km and temperature above 80°C. At depths >4.6 km with corresponding temperatures >190°C, the late stage began, characterized by the presence of R>1 I-S with <10% smectite.