A theoretical model is presented to describe the adsorption of surfactant molecules at the water/oil interface for the case when transport across the interface must be considered due to the solubility of the surfactant in both liquid phases (aqueous solution drop in an oil phase). The theory is based on a diffusion-controlled adsorption model and a numerical difference scheme is used to solve the problem for closed finite systems. By simulating adsorption-desorption processes, the influence of the geometrical and thermodynamic parameters on the dynamic adsorption behavior is studied. In particular, the effects of the volume ratio, surfactant concentration, diffusion coefficients, and distribution coefficient on the process are analyzed. The model allows quantitative interpretation of the experimental data obtained for an alkyldimethylphosphine oxide at the water/hexane interface. The interfacial tension signals, presenting a minimum under some conditions, are fitted by the theoretical curves.