This paper discusses the viscosity characteristics of ash fractions produced from the co-combustion of coal and biomass in a pilot-scale pulverized fuel (PF) boiler and from the incineration of municipal solid waste (MSW) in a Danish incinerator that were determined using the high-temperature rotational viscometer method. Two different Setups in the United States and Denmark were purchased by the same company, HAAKE, but different versions of the basic instrument were used in the study to determine the reproducibility of the method. The two sets of measurements show that the method generates, in principle, reproducible results. The comparison of the generated results as well as the operation and measuring capabilities of the two setups provide useful information regarding the interpretation of the viscosity data as well as the capabilities and the limitations of the method and the specific setups.