Abstract: The equilibrium constants of the extraction, the molar ratio of butyl acetate and dioctyl sulfoxide to penicillin G in the extracts, and the bonding structures of the extracted species were studied with chemical analysis as well as the absorbance shift of Fourier transform-infra red spectra of the functional groups in penicillin G. Reverse micelles were observed in the butyl acetate solution containing penicillin G. The diameters of the reverse micelles could be as large as 0.6μm.