Abstract: A method for increasing the effectiveness of a biocide is described. In the method, at least one biocide and at least one diethanolamide are applied to a substrate or aqueous system subject to the growth of microorganisms. The diethanolamide is applied in an amount effective to increase the biocidal activity of the biocide. Biocidal compositions are described where the biocide and the diethanolamide are present in a combined amount effective to control the growth of at least one microorganism. Methods for controlling the growth of microorganisms on various substrates and in various aqueous systems are also described. The combination of the biocide and the diethanolamide is particularly useful as a biocide in the leather industry, the lumber industry, the papermaking industry, the textile industry, the agricultural industry, and the coating industry, as well as in industrial process waters.