Equil. and dynamic interfacial tension was detd. with a drop vol. tensiometer for equimol. trioctylphosphine oxide (I)-di-2-ethylhexylphosphoric acid (II), and equimol. I-Alamine 336 (trioctyl/decylamine as active compd.) (III) mixts. studied as extractants in the water-toluene system. From each mixt. the extractants were co-adsorbed at the water-toluene interface but they were not synergistic in suppressing the interfacial tension. The compn. of the adsorbate differed from that of the bulk phase. Adsorption kinetics was controlled by the dominating extractant. In the binaries, I was surprisingly inactive as adsorbent The surface of the mixed adsorption monolayer was occupied in 30% by I mols. Mol. interaction parameters, adsorption free energy, Szyszkowski's isotherm correlation coeffs., diffusion coeffs., and surface area for a statistical mol. adsorbed were detd.