The purpose of this paper is to investigate the effect of monovalent salts on the thermodynamics of micelle formation in aqueous C-8-lecithin solution. The critical micelle concentration (CMC) and mean hydrodynamic radius (<(R)over bar (h)>) of the micelles as functions of amphiphile concentration, temperature, ionic identity, and strength were determined using laser light scattering technique. Then the standard Gibb's free energy (Delta G(0)), enthalpy (Delta H-0), and chemical potential (Delta mu) of the micelle formation and growth were deduced from the experimental data. It is found that the micellization is a spontaneous process. Compared with the situation in a salt free solution, the reaction of micelle formation in KI added solutions is less exothermic, and the micelles in the solutions are less likely to form, and are smaller and less stable. The reaction of micelle formation in KCl added solutions, however, is more exothermic, and the micelles in the solutions are facile to form, and are larger and more stable. This indicates that monovalent salts affect the micelle formation and growth in the way predicted by the tendency of Delta mu to vary with ionic identity and strength of the salts. (C) 1997 American Institute of Physics.