Abstract: Interfacial emulsification often occurs in the extraction of biological fermentation liquors containing organic acids. As a result of theoretical analysis and experimental work, certain substances present in the fermentation liquor have been identified as the cause leading W/O emulsions. Emulsification caused by surface activity of certain extractants was also examined by using microphotography for both W/O and 0/W emulsions, and a general discussion was then presented on the three phenomena of emulsification, third phase formation and interfacial contamination in extraction processes.