The temperature-composition dependence along the Krafft boundary in surfactant-water systems and the effect of the third component on the surfactant precipitation temperature are analyzed on the basis of strict thermodynamic relationships and using approximate models (the ideal monodispersed micellar solution approximation, the pseudo phase separation model). As it follows from the consideration a substantial decrease in the surfactant precipitation temperature (Delta T) may be observed on addition of light amphiphilic substances forming mixed micelles with the basic surfactant, the distribution coefficient between the micelles and the aqueous surroundings being an important characteristic of an additive. Regularities in the dependence of the Delta T-effects on the third component chemical nature and on the mixture composition are discussed. The model approaches are illustrated in application to the N-dodecanoyl-N-methylglycamine-water system and for the system containing n-butanol as the third component. For the ternary system the concentration dependence of the alcohol activity coefficient in the aqueous pseudo phase was taken into account with the help of the UNIFAC model, this dependence influencing seriously the calculated results. The estimated Delta T-effects are in a satisfactory agreement with the experimental data, an information on the mixed micelles composition is drawn from the calculations.