Abstract: The present invention is directed to a method for determining the surface activity properties of an amphiphilic substance, such as a drug, the method including a step wherein the surface tension of an aqueous solution of the said substance is measured at its air-water interface at a plurality of concentrations of said substance and determining the relationship between the surface tension and the concentration of the substance, and using the relationship so determined to predict the surface activity properties of the substance, according to which method, to the aqueous solution, a water soluble substance, which increases the surface tension of the aqueous solution is added in an amount to provide a concentration of 0.3 M up to the saturation concentration of the said substance in said solution.