AbstractThe viscosity of NaF ‐ AlF3mixtures has been determined in the range 0‐35 mol‐% AlF3, 9501114°C using a recently developed oscillating cylinder viscometer. The method is absolute, with an estimated accuracy of 0.5 ‐1% SD in routine work. ‐ A model is presented that reproduces the measured viscosities within 0.55% SD. It assumes the presence of F−, AlF63‐, and AlF4−, and the Arrhenius temperature and mixture rules were used to fit model parameters to the