In the present paper, we report the micellization at different fixed temperatures [(293.15, 303.15, 313.15, and 323.15) K] and the clouding behavior of the phenothiazine drug 10-[2-(dimethylamino)propyl]phenothiazine hydrochloride (promethazine hydrochloride, PMT) in the absence and presence of KCl. The critical micelle concentration (cmc) of PMT was measured by the conductivity method. The cmc values decrease with increasing the KO concentration, whereas with increasing temperature, the cmc values increase. The thermodynamic parameters, namely, the standard Gibbs energy (Delta(m)G degrees), standard enthalpy (Delta(m)H degrees), and standard entropy (Delta(m)S degrees) of micellization of PMT were evaluated, and they indicate greater stability of the PMT solution in the presence of KCl. PMT shows phase separation also. The cloud point (CP) of PMT decreases with increasing pH because of deprotonation of the drug molecules. The CP values increase with increasing KCl as well as PMT concentration because of micellar growth. Furthermore, the thermodynamic parameters were evaluated at the CP.