The standard Procedure for determining the anionic surfactant content of aqueous and wastewater effluents is to measure the concentration of methylene blue active substances (MBAS). A methylene blue active substance is a material that forms an association complex (ion pair) with methylene blue in an aqueous phase, which can they be extracted as the association complex. Many substances are methylene blue active; however, anionic surfactants are some of the most prominent.1 Alkyl sulfonates (RSO3-), alkyl sulfate esters (ROSO3-), and alkyl benzene sulfonates (RC6H6SO3-) are strongly ionized anionic surfactants that form association complexes with methylene blue in aqueous solution. The resulting ion pair complexes can be extracted from aqueous solution into chloroform with nearly quantitative efficiency by a single extraction.2