Numerous paleosols occur in the Lower Visean peritidal limestones of eastern Belgium. At the top of the peritidal unit, a widespread paleosol horizon occurs which has been analysed for its carbon and oxygen isotopic composition. In spite of the Paleozoic age of the pedogenic carbonates, the influence of evaporation and soil-ps CO2 on the stable isotopic composition of the paleosol is very pronounced. This has been assigned to the long exposure time in a semi-arid seasonal climate. Evaporation caused an enrichment in O-18 in the paleosol compared with the over- and underlying limestones, which were cemented and mineralogically stabilized in meteoric pore waters. The influence of soil-PS CO2 in the soil formation is clearly indicated by the depletion in C-13 of the pedogenic carbonates (deltaC-13 = - 5 parts per thousand(PDB)) compared with the carbon isotopic composition of Lower Visean marine limestones (deltaC-13 = +4 parts per thousand(PDB)). The possible effect of carbon derived from atmospheric CO2 on the deltaC-13-values of the paleosols is discussed.