Data of cation-exchange capacity (C.E.C.) were obtained for 22 soil profiles of the State of São Paulo. C.E.C. of organic matter varied from 190 to 400 me. per 100 g in the samples from surface horizons, and from 160 to 490 in those from other horizons. A highly significant relationship was obtained between organic C.E.C. and soil pH, for the soil samples from surface horizons (r++= 0.81) and also for the horizons collected just below the surface horizon (r++ = 0.76). Clay C.E.C. varied between 2.7 and 20.0 me. per 100 g in 21 of the soil profiles studied. It was not possible to relate the clay C.E.C. values to the mineralogical composition, as determined by X-ray diffraction. As average, the contribution of organic matter to the C.E.C. in 21 soil profiles was: 74 per cent for surface samples; 58 per cent for the A2 and A3 horizons; 47 per cent for the B1 horizon; and 35 per cent for the B2 horizon.