The viscosity of the NaCl + AlCl3 melt system has been measured in the composition range 48.5-59 mol % AlCl3 and temperature range 200-400-degrees-C. The viscosity of the system is expressed as a function of temperature and composition. The effect of dissolved sodium oxide on the viscosity of the system was also measured up to a nominal oxide concentration of 0.5 mol kg-1, as a function of temperature, Na:Al ratio, and oxide content.