The conditions under which the two black films—the first of greater and variable thickness and the second of about 44 Å—are formed by solutions of sodium dodecyl sulphate in the presence of NaCl are established and the effect of several additives upon the relative stability of these two films is investigated. Specific effects of counterions upon this behaviour are marked and are interpreted in terms of an ionic size effect upon the structure of the second black film and of preferential surface adsorption. Some conclusions about the composition and heat of formation of the second black film are drawn using Le Chatelier's principle. At higher ionic strengths counterions have also specific effects upon the thickness of the first black film in line with their hydrated size and adsorption.