A surface tension method developed previously has been used to investigate the formation of inclusion compounds between dimethyldodecylamine oxide (DDAO) and beta-cyclodextrin, at pH values in the range 2 to 8, in the absence and in the presence of added 0.10 M NaCl. Thermodynamic activities of the DDAO are inferred from measurements of surface tension at concentrations lower than the critical micelle concentration of the surfactant. Binding constants are determined directly from the effect of added cyclodextrin on the surface tension vs concentration curves for DDAO in water and in the 0.10 M NaCl solutions. Binding effects are discussed in relation to the charge of the DDAO head group, which can be controlled by varying the pH, and the screening effect of the added electrolyte. (C) 1994 Academic Press, Inc.