To investigate the contribution of organic matter to the cation exchange capacity (CEC) and the specific surface area (SSA) of organomineral complexes in soils, we studied A horizons of four soils (two Hapludalfs and two Argiudolls) in central Iowa. The effect of mineralogy on CEC and SSA was held nearly constant by sampling soils developed in the same parent material (loess) and landscape position (summit). A range of organic matter contents was obtained by variations in native vegetation, the effects of cultivation, and horizon depth. The CEC, SSA, and organic C content of unfractionated samples were determined. Soil samples were also separated into coarse silt, fine silt, coarse clay, and fine clay size fractions after dispersion by sonification. The organic C content of each fraction was determined, as were CEC and SSA, before and after peroxide treatment. Multiple linear regression equations were developed to relate CEC and SSA to organic C and peroxidized SSA. Partial regression coefficients suggested that the net contribution of organic matter to the CEC of unfractionated soil materials was 184 cmol(+) kg−1of organic C. In the coarse silt, fine silt, and coarse clay fractions, organic matter was estimated to contribute approximately 559 cmol(+) kg−1of organic C to the CEC and 7.22 x 105m2kg−1of organic C to the SSA of the fractionated materials. On average, organic matter was calculated to contribute 49% of the CEC and 19% of the SSA of the fractionated materials. Because “independent” variables were themselves highly correlated, principal components regression analysis was used to improve the precision of estimates of organic matter contributions derived from the partial regression coefficients.