Many solutes, either naturally occurring or introduced, are surface active and sorb preferentially at the interfaces of subsurface systems. In multiphase systems, the sorption of surfactants affects the capillary pressure-saturation relationships, fundamental constitutive relationships in the modeling of multiphase flow. In this study, the impact of surfactant sorption on capillary pressure relationships for organic liquid-waters systems was demonstrated by qualitatively correlating measurements of sorption and zeta potential, with interfacial tension and contact angle and, in turn, quantitatively relating these measurements to changes in capillary pressure-saturation relationships for o-xylene-water-quartz systems containing a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The results show that the sorption of CTAB causes the naturally hydrophilic system to become hydrophobic, as evidenced by a change in the contact angle from about 10¿--15¿ to 155¿ or 180¿, depending on the pH. This change in hydrophilicity is reflected in the zeta potential of the system which goes from negative to positive as the aqueous phase CTAB concentration increases. The spontaneous imbibition capillary pressure-saturation relationship is more sensitive to the sorption of CTAB than the drainage relationship. To predict the observed changes in both capillary pressure-saturation relationships, a modified form of Leverett's function was used where roughness and curvature corrections were incorporated into the intrinsic contact angle to give an operational contact angle. A comparison of the measured and predicted capillary pressure-saturation relationships showed reasonable agreement.