The inadequacy of the usually used film theory is shown by fitting experimental data of the zinc extraction with di(2-ethylhexyl)phosphoric acid in the presence of a planar interface. It is explained why it is necessary to model the mass transfer carried out by an unsteady diffusion process. By introducing the flow-dependent number of phase contacts as a function of the Reynolds number the phase contact time is described theoretically. In this way the approximation of the penetration model and the film penetration model are solved analytically. Subsequently, the model equations are validated by using experimental data.