The use of solvent extraction (SX) and supported liquid membrane (SLM) for the separation of citric and lactic acids in aqueous solutions were studied. Experiments were carried out as a function of the total acid concentration, the concentration ratio of the acids, and the concentration of extractant tri-n-octylamine (TOA). In SLM systems, the effects of temperature and strip-phase composition were also examined. It was shown that the equilibrium distribution of SX of each acid was greatly affected in the presence of the other. In addition, the competitive transport of the acids in SLM was explained from the kinetics of SX. Separation factors based on SX distribution and SLM transport were defined and used to optimize conditions for effective separation.