An apparatus for measuring the relative viscosity of molten glass‐forming oxides at elevated temperatures and pressures is described. It utilizes a concentric cylinder viscometer and electric furnace enclosed in a steel pressure bomb and is gas pressured. Four thermocouples and three heating circuits enable the temperature of the sample to be controlled to within ±0.2°. Data on the pressure dependence of viscosity of molten boron trioxide in the temperature range of 380–470°C and at pressures up to 300 bars are presented.