A theoretical study on the sorption kinetics at liquid-liquid interfaces of surfactant component soluble in both phases is presented here. The diffusion process in bulk phases is considered as well as the presence of an interfacial potential barrier to adsorption and desorption. In the present paper, a generalization of a recent study on the sorption kinetics at a fluid interface with a surface-active component soluble in only one phase [F. Ravera et al., J. Colloid Interface Sci. 156, 109 1993)] is given. The new approach consists of writing a renormalized diffusion equation, equivalent to the kinetic sorption equation, by introducing renormalized diffusion coefficients and boundary conditions suitable for the bulk concentrations. The treatment leads to a relationship between the surface excess concentration and tile time, in terms of the interfacial potential barriers of adsorption and desorption. (C) 1994 Academic Press. Inc.