The dependence of a dye's spectral behavior on a surfactant's concentration has been exploited for almost fifty years to estimate the critical micelle concentration (CMC). In the present study, four models of dye-surfactant interaction have been employed to determine the extent of involvement of several factors in the technique. The results indicate that a spectral change CMC (T-CMC) can be less than, equal to, or greater than a CMC obtained for the same system by a physical measurement (P-CMC). A decrease in T-CMC can result from one or more of the following changes: an increase or decrease in dye concentration; an increase in the solubility of dye surfactant salt; an increase in micelle tendency to solubilize dye; an increase in the number of dye ions which a micelle can accommodate; an increase in the inclination of surfactant and dye ions to form induced mixed micelles. Although dye concentration has some effect on the P-CMC, it is relatively small. (C) 1994 Academic Press, Inc.