To confirm the applicability for the extraction of penicillin G by an emulsion liquid membrane (ELM), the degree of decomposition of penicillin G during extraction was theoretically calculated. Decomposition was less than 1% provided that the initial sodium carbonate concentration in the internal phase was correctly determined, which proved the applicability of the ELM process. The procedure to determine the initial carbonate concentration in the internal phase was also described in order that the pH in the internal phase should be within the relatively stable range for penicillin G at the end of the extraction.