Isopiestic molalities of aqueous solutions of the alkali metal hydroxides have been measured at T = (383.25, 413.12, and 443409) K. The isopiestic standard for calculation of osmotic coefficients was NaCl(aq). The ion-interaction model gave an excellent fit with the experimental osmotic coefficients and provided parameters for the calculation of activity coefficients. Osmotic and activity coefficients both decrease with increasing temperature in this temperature range. As at T = 298.15 K, the order of the results is the reverse of that for the alkali metal chlorides. The model of Simonson et al. (J. Chem. Thermodynamics 1989, 21, 561-584) is an excellent predictor of the experimental results for NaOH(aq). A simple model is presented from which activity and osmotic coefficients (at p(sat)) can be calculated as a function of molality for T = 273.15 K to T = 523.15 K.