The viscometer described in this note is of general applicability but its features are particularly suited for measurements with molten salts having appreciable vapor pressures and/or chemical reactivity to atmospheric moisture. While sealed viscometers are not unknown [1, 2], the present design ensures a constant starting volume and minimizes some of the inherent experimental difficulties, for example, calibration, repeated measurements, and kinetic energy and surface tension corrections. Magnitude and sources of errors are briefly considered, and results with this viscometer for KN03(mp, 337°C), HgCl2(mp, 277° C; bp, 304°C), and CsN03(mp, 414°C) are reported.