AbstractThe viscosity of NaCl‐AlCl3melt mixtures has been determined in the range 0.50 ≤X AlCl 3≤ 0.80, 160°C to 300°C. The viscosity of pure AlCl3was measured over the temperature span 196°C to 286°C. A recently designed oscillating vessel viscometer was used. The method, which utilizes closed cylindrical cups subjected to vapour pressures exceeding 11 atm, is absolute and requires no calibration against reference liquids. Results are corrected for surface effects and for evaporation within the crucible. Applying Arrhenius temperature dependencies and the Arrhenius mixing rule, the viscosity is expressed as a function of temperature and composition with an overall standard deviation of 0.4% when the presence of AlCl−4, Al2Cl−7, Al3Cl−10, a