This paper deals with a selective spectrophotometric determination of dibucaine and procaine, local anesthetics, by using thermochromism of ion-associate. Tetrabromophenolphthalein ethyl ester (TBPE) is an excellent extraction reagent for quaternary ammonium ions and amines. TBPE forms red or blue ion-associates with amines or quaternary ammonium ions. Of these ion-associates, the red extract shows a reversible thermochromism. The rate of decrease of the absorbance with temperature change (ΔA/Δt) is proportional to the concentration of the amine in the aqueous phase (ΔA=A25°C-A50°C, Δt=50°C-25°C). Consequently, a straight line was obtained in the concentration range (1-5)×10-6M dibucaine and procaine in the final aqueous solution, with ΔA/Δt as the ordinate and the concentration as the abscissa. Though the ΔA/Δt value of TBPE-dibucaine and -procaine associates were somewhat affected by ΔA/Δt of (5-10)×10-7M each quaternary ammonium salt, a good recovery could be obtained by subtracting the ΔA/Δt value of each salt measured previously from the ΔA/Δt value of the mixture. This method is useful for the determination of amines added to the human urine.