The viscosity of the molten system Na3AlF6 + AlF3 + KF + (Al2O3) was investigated using a torsional oscillating cylinder viscometer. The melts contained up to 25 mass % AlF3 and 5.85 mass % KF. The temperature ranged from 865 to 1030 degrees C. The method was absolute, and the accuracy of the viscosity data of cryolite-based melts was better than 3%. The product eta V where eta is the viscosity and V the molar volume can be described by the equation eta V/(mPa . s . cm(3) . mol(-1)) = 800.13 - 0.585t/degrees C - 3.3141w(AlF3) + 25.027w(KF) - 35.614w(Al2O3) - 1.726w(AlF3)w(KF) + 2.577w(AlF3)w(Al2O3) where w(AlF3) denotes the mass percent of excess aluminum fluoride with respect to cryolite.