BACKGROUND: The Cyanex (R) 923 (trialkylphosphine oxides, TRPO)-n-heptane/cerium(IV)-H2SO4 extraction system has been investigated focusing on the physicochemical properties, surface active species and interfacial phenomena. The effects of H2SO4 and Ce(IV) extraction on them were considered. RESULTS: Results showed that the density and refractive index reflect the mass transfer by H2SO4 and Ce(IV) extraction and the change of refractive index was more sensitive than density. The interfacial tension decreased on extraction of H2SO4 but increased on extraction of Ce(IV). The viscosity of the equilibrium organic phase increased abruptly when the extracted H2SO4 concentration in the organic phase reached certain high values. The formation of reversed micelles, with mean diameter of about 10nm, at high H2SO4 concentrations in the organic phase, is suggested by various measurements such as viscosity, interfacial tension and dynamic light-scattering (DLS). CONCLUSION: It is suggested that TRPO-H2SO4 Complexes are more surface-active than TRPO itself and tend to aggregate into reverse micelles by self-assembling in the organic phase but the Ce(IV)-TRPO complexes are neutral, less surface-active than TRPO and not helpful for reverse micelle formation. (C) 2008 Society of Chemical Industry.