A diffusion-type hydrogen-activity meter has been developed at Argonne National Laboratory to measure the hydrogen level of the sodium coolant in Liquid Metal Fast Breeder Reactor systems. The meter can be operated in two modes: an equilibrium mode and a dynamic mode. In the equilibrium mode, the hydrogen pressure in equilibrium with the sodium is measured by a pressure sensor and is related to the hydrogen concentration in the sodium by the Sieverts’ law constant for the hydrogen-sodium system. In the dynamic mode, the hydrogen concentration in sodium is measured by the rate of hydrogen diffusion through a nickel membrane immersed in the sodium. A vacuum of 10−6 to 10−8 Torr is drawn on the membrane at a steady rate by an ion pump, and the hydrogen activity gradient from the sodium side to the vacuum side of the membrane causes hydrogen to diffuse through the membrane. The partial pressure of hydrogen on the vacuum side, a measure of the hydrogen flux and hydrogen activity in the sodium, is determined by measuring the current to the ion pump. The meter is unique in that it does not require any form of external calibration. Data were obtained in this study of the Sieverts’ constant and hydrogen solubility for the range of 0.03 to 1 ppm. The Sieverts’ constant is slightly affected by temperature over the range 370 to 500° C and is given by The solubility of hydrogen in sodium for 0.03 to 70 ppm (including the data of others) is given by