Isopiestic molalities of aqueous solutions of the alkali metal bromides have been measured at T = 383.15 K to T = 498.15 K. Aqueous NaCl served as the isopiestic standard for the calculation of osmotic coefficients. The ion-interaction model gave an excellent fit to 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. 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. Comparison of the stoichiometric activity coefficients with those of the corresponding alkali metal chlorides reveals quite complicated relative behavior.