The influence of adsorption of low amounts (1.00 x 10(-8)-2.33 x 10(-5) mol.L-1) of Triton X-100 and hydroxyl ion (appropriate to the practical pH range of 6.00-9.37) on the interfacial tension of two recommended systems for liquid liquid extraction investigations, toluene water and cumene water, at 25.0 degrees C is studied. Solutions with a maximum relative standard deviation in concentration of 1% have been used; meanwhile, the uncertainty for measured interfacial tensions is +/-0.03 mN.m(-1). From the data and in agreement with the commonly used Szyszkowski model, a much higher adsorption effectiveness, but a lower adsorption tendency, for Triton is concluded. The influence of Triton in lowering interfacial tension is significantly weakened by hydroxyl ions. Meanwhile, the estimated interfacial composition and interaction parameter (using the theory of nonideal interactions in binary mixtures) are strongly dependent on the bulk mole fraction. The hydroxyl ion shows maximum adsorption at the bulk mole fractions around 0.50 and 0.60, where a high rate of mass transfer could be expected.