Coverage of clay surfaces by soil organic matter (SOM) may limitthe efficacy of the soil mineral fractions for adsorption of organic contaminants and pesticides. Two methods were scrutinized for quantitatively assessing the availability of clay surfaces in a smectitic Webster A-horizon soil for sorption of p-nitrocyanobenzene (p-NCB) and diuron. One method, described previously, involves the summation of independent contributions of SOM and swelling clays to sorption of organic solutes. For this method, several assumptions must be made and/or procedural difficulties overcome in the determination of certain terms in the equation proposed for calculating the fractional availability of mineral surfaces (fa). To alleviate the methodological limitations, we developed an alternative approach for determining fa. Good agreement between fa values was obtained from both methods for p-NCB but not diuron. For p-NCB sorption, fa values varied between 0.55 and 0.71. For diuron sorption, our alternative equation estimated fa values varied between 0.41 and 0.61; the other approach yielded negative values. The results demonstrate that SOM does reduce the availability of clay surfaces, hence, suppressing sorption by the Webster A-horizon soil. Our newly developed method provides more reasonable estimates of the availability of soil-clay surfaces for sorption than an earlier published approach.