The apparatus described is used in the measurement of the electrical conductivity of vapor phase systems at temperatures to 400°C and pressures to 4500 psi. A small, lightly constructed, platinum‐iridium conductivity cell is enclosed in a larger, heated, steel pressure vessel. The pressure developed in the conductivity cell on heating is counterbalanced by admitting nitrogen into the pressure vessel. The direction and magnitude of the pressure differential between the vapor in the cell and the surrounding nitrogen is indicated by a meter circuit which measures the potential drop of a radiofrequency current across a variable condenser which is operated by an aneroid diaphragm on the cell. Admission and release of nitrogen is controlled manually. Measurements of the cell resistance are made by conventional ac bridge methods and are not described.