Clay surface coverage by soil organic matter may limit the efficacy of the soil clay mineral fraction for the adsorption of organic contaminants and pesticides. Two methods were used to evaluate the availability of clay surfaces in a (smectitic) Webster soil for sorption of para-nitrocyanobenzene (a solute known to be strongly sorbed by smectite clays) and diuron. One method described previously (but not tested) involves the summation of the independent contributions of soil organic matter and swelling clays to the sorption of organic solutes by soil. However, to experimentally determine the fraction of the clay mineral surface available for sorption within soil several assumptions must be made and procedural difficulties overcome in the determination of certain terms in the equation used to calculate fractional availability. We developed an alternative approach for determining fractional availability that alleviates these methodological limitations. We found good agreement between fractional availability values obtained from both methods of analysis for nitrocyanobenzene but not for diuron. Generally, for nitrocyanobenzene sorption the fractional availability values ranged between 0.6 and 0.7. For diuron sorption, using our alternative approach, fractional availability values varied between 0.28 and 0.47; the other approach yielded negative values. These results indicate that soil organic matter does supress sorption of nitrocyanobenzene and diuron by Webster soil clays by 30 to 40% and 53 to 72%, respectively.