The transport rates of lactic and citric acids from binary solutions across supported liquid membranes containing tri-n-ocytylamine are measured. A mass transfer model is proposed which considers aqueous film diffusion, interfacial chemical reaction, and membrane diffusion. It is shown that the difference between the measured and modeled rates was more significant at high total acid and amine concentrations. This could be partly due to the ignorance of mutual interactions between the acids in the aqueous phase and the complexes in the membrane phase in this model (chemical effect), and to limited validity of "fixed" transport properties over a rather wide concentration range (mechanical effect).