Measurements were made of the viscosity of Columbia River basalt and basalt-urania melts containing up to 18 wt% UO2 in the temperature range 1360 to 1960°C. A concentric cylinder-type viscometer was used. The combined data were fit by the equation, where η is viscosity in poise, T is temperature in degrees Kelvin, and X is the weight percent UO2 in the melt.