The interaction of benzoic acid and a series of hydroxy, amino, chloro, and nitrobenzoic acids with surfactants of the quaternary ammonium type was studied by means of viscosity measurements. Only salicylic acid was found to interact with the surfactants leading to an increase in viscosity which reached a maximum at approximately the concentration at which the surfactant solution became saturated with sarlicylic acid. Apparently the interaction is specific and is limited to the ortbo hydroxy substitution of benzoic acid, as no viscosity effect was observed with the ortbo, meta, and para isomers of amino, chloro, and nitro substituted benzoic acids. In addition, sodium lauryl sulfate and cetomacroeol 1000 did not exhibit viscositv changes in the presence of salicylic acid. The viscosity effect was not related to pH. The interaction of benzoic acid and a series of hydroxy, amino, chloro, and nitrobenzoic acids with surfactants of the quaternary ammonium type was studied by means of viscosity measurements. Only salicylic acid was found to interact with the surfactants leading to an increase in viscosity which reached a maximum at approximately the concentration at which the surfactant solution became saturated with sarlicylic acid. Apparently the interaction is specific and is limited to the ortbo hydroxy substitution of benzoic acid, as no viscosity effect was observed with the ortbo, meta, and para isomers of amino, chloro, and nitro substituted benzoic acids. In addition, sodium lauryl sulfate and cetomacroeol 1000 did not exhibit viscositv changes in the presence of salicylic acid. The viscosity effect was not related to pH.