The batch extraction of penicillin G from a model media was studied so as to obtain the optimal w/o ratio in an emulsion liquid membrane (ELM) system with the help of our previous works. First of all, the effects of organic solvents or surfactants in membrane phase on apparent degree of its extraction at various w/o ratios were investigated. The organic solvents and surfactants were kerosene/n-butyl acetate and Span 80/PARABAR 9551, respectively. The optimal composition between two surfactants for the highest apparent degree of extraction was obtained at each w/o ratio. With the optimal surfactant composition at each w/o ratio, citrate buffer solutions of three different concentrations having about pH 4.8 were selected as the external aqueous phase. The mass of Na2CO3 in the internal phase was kept constant for every w/o ratio in order to investigate the effects of w/o ratio on the degree of extraction under the same trapping mass of Na2CO3 for penicillin G. The highest possible concentration of Na2CO3 in the internal phase was chosen using balance equations and equilibrium expressions. In consequence, the highest actual degree of extraction of 98.2% and the lowest percentage of swellings of 36.0 were obtained in the specific ELM system with 0.41M citrate buffer solution, 0.175M Na2CO3 and w/o ratio of 1/1.