Dolomitization of carbonate rocks of Irati Formation (Permian) from São Paulo State is studied in the present papel. Outcrop samples as well as drilling ones are almost always made up of dolostones, either pure or slightly calcitic, independent of texture or structure. These facts indicate that the idea of syngenetic dolomitization is correct. Several different structures confirm that this process preceeded the complete diagenesis of carbonatic sediments. The phenomenon resulted of the refluxion of high1y concentrated and dense brines whose Mg/Ca relation was highly increased. These brines flowed slowly from the margins toward the center of the depositional basin. The carbonate rocks show higher calcite content in the occurrences affected by diabase sills, whose high temperature of intrusion associated with heated aqueous vapor promoted the dedolomitization phenomenon. The magnesium so expelled was probably incorpo rated in montmorillonite, the main c1ay mineral of the shales just above the dolomitic basal stratum.
