The adsorption of a surfactant to solid surfaces can lower the concentration of free surfactant in aqueous solutions. If the degree of adsorption is high, then surfactant concentration could drop below the CMC (critical micelle concentration), rendering the surfactant solution unable to solubilize. Therefore, the amount of surfactant mass that will sorb should be accounted for when selecting injection concentration. In this study, a nonionic surfactant, Triton X-100, was used to study the sorption characteristics of diesel on to soil below and above CMC. The surface tension is used for calculating this kind of sorption. The results indicated that when the concentration of surfactant was lower than the CMC, the amount of surfactant sorbed on soil increased with increasing surfactant concentration, and the amount of desorbed diesel was relatively low compared to the original amount added to the soil, but that the amount of diesel desorbed from soil at surfactant concentrations above CMC was relatively high. © 2010 EPP Publications Ltd.