This study describes an experimental technique to determine the viscosity of high-temperature ash samples, using a thermomechanical analysis (TMA) apparatus. The experimental technique was validated at low temperature by measuring the viscosity of a synthetic oil and comparing its viscosity with the calibrated value. Validation at high temperatures was achieved by comparing the measured viscosity of ash samples with the viscosity measurements obtained using a conventional high-temperature rotating bob viscometer. The results indicate that the technique can rapidly provide an indication of the viscosity of slags at high temperatures and could prove to be an alternative, cost-effective technique to current high-temperature ash sample viscosity measurement techniques.