SummaryIn 24 soils the CEC of untreated samples, of samples treated with hydrogen peroxide, and of samples treated with dimethyl sulphate, was determined at pH 3, S and 8. The CEC of clay and organic matter was calculated from multiple regression equations and compared with values after treatment with peroxide and after methylation. At pH 3 and pH 5 the CECs of clay by the three methods were similar, whereas at pH 8 they were significantly lower on untreated samples.The results are interpreted in relation to the formation of organo‐mineral complexes and to the interaction of the acid functions of the clay and organic matte