Abstract: An improved model for liquid membrane mass transfer has been developed. This model takes into account both the diffusion resistance of the membrane phase in non-steady state and the diffusion resistance of the external phase boundary film and the membrane leakage. An approximate analytical solution of non-linear equations was obtained by using pertubation method. This model was examined with the experiments in which molybdenum was extracted by batch liquid membrane method. The results showed that when the concentration of the external phase was higher, the diffusion resistance of the external phase boundary film could be neglected. otherwise, the resistance should be taken into ccount. In particular, the factor ot membrane leakage had greater influence on the later stage of the reaction. The modeling computation agreed well with experimental data.