The viscosity of liquid Fe78S22 was measured up to 16 GPa and 1723 K using a high-speed CCD camera system (125 frames/s maximum). In order to prevent a chemical reaction between the sample and a Re viscosity marker sphere, the Re spheres were coated with alumina to a thickness of 1-2 μm. The measured viscosity coefficients were 3.2-8.5 mPa-s and the activation volume was estimated to be 1.46 cm3/mol. The viscosity of liquid Fe86 C14 was also measured up to 4.5 GPa and 1843 K. The viscosity coefficients were 3.8-6.3 mPa-s. There was no large difference in viscosity coefficient and activation volume between the Fe-S and Fe-C eutectic liquids in the range of measurement. Viscosities of Fe-S and Fe-C eutectic liquids are likely to remain low in the planetary interior.