It was found during the oxidation of methylthio substituted pyrimidines and 1,3,5-triazines that pyrimidinesulphones were obtained in better yield than triazinesulphones because the oxidation is influenced by the structure and substitution of heterocycles and it could not were selectively stopped at this level. Depending on the pH and solvent, the methylsulphonyl and the formed methylsulphonium groups can rapidly take part in nucleophilic substitution reactions resulting in RO-triazine and -pyrimidine derivatives (R = H, CH3). These reactions may be utilised in such processes where the methylthio substituted heterocycles are formed as by-products. After their oxidation, the nucleophilic substitution leads to the desired clean products.