The oil-water interface is thought to have a significant role in the extraction of proteins into nonionic microemulsions. Several surfactants, that have a known specificity for particular proteins, have been tested for their ability to improve the specificity of Neodol 91-2.5 (linear alcohol ethoxylate) microemulsions. Only some of these protein-specific surfactants were capable of improving specificity. In successful extractions, a minimum amount of protein-specific surfactant has to be added to Neodol 91-2.5 nonionic microemulsion before any protein is extracted, Above this minimum, all of the protein is removed, It was also observed that below this minimum the protein-specific surfactant is present at the oil-water interface, but is not available for protein extraction, This is in contrast with affinity-based reverse micellar extraction where the appearance of the ligand at the oil-water interface is sufficient to cause protein extraction, The physical structure of the interface plays a role in the protein-surfactant interactions seen in nonionic microemulsions, Two criteria need to be satisfied as a basis for successful protein extraction. A minimum headgroup area of the protein-specific surfactant, and a minimum fractional area coverage of the protein-specific surfactant at the oil-water interface, are simultaneously required for protein extraction, A conceptual model has been proposed to explain these observations.