A mathematical model is proposed to describe the effects of boundary layer resistance and interfacial reaction on the time lag in supported liquid membranes for metal ion separation. The model shows that the presence of boundary layer resistance and interfacial reaction delays the time-lag, compared with the limiting case that rapid equilibrium at the interfaces and negligible boundary layer resistance are assumed. Calculated result describes the expected trend and the model equation allows one to predict the lumped parameters which represent the ratios of the diffusion time in the membrane to the characteristic time for boundary layer transfer and interfacial reactions.