The thermal maturity of Paleozoic rocks in the SE part of the Bohemian Massif is characterized by clay minerals and organic matter. The expandability of illite-smectite (S in I-S), illite crystallinity index (IC) and reflectance of (R_r) were measured and their regional distribution was evaluated. The mutual correlation of IC and R_r from diagenesis to very low-grade metamorphism is compared with the published data and used to distinguish data with more reliable paleogeothermal information from those affected by other factors. In the SE part the Paleozoic units have illite-smectites with an expandable component of 15–35 % S. The reflectance values (Rr of 0.55–1.1 %) are in good agreement with the expandability and suggest the oil window range of catagenesis with paleotemperature close to 100 °C. In the NNW part of the area the clays contain no expandable layers in illite. The illite crystallinity (IC of 0.36–0.24 Δ°2Θ) and vitrinite reflectance (R_r from 3.17 to 5.23 %) indicate very low-grade metamorphic conditions with probable maximum paleotemperatures of 240–300 °C. The systematic change in both clay and organic parameters shows the gradual decrease in thermal exposure towards the front of the Variscan orogenic zone in the S and SE and suggest extensive erosion in the NNW.