Monitoring of hydrogen in liquid sodium with a diffusion tube type hydrogen detector has recently become of interest in the liquid metal fast breeder reactor (LMFBR) development program. The present investigation was undertaken to characterize a diffusion type detector and to study sodium-hydrogen chemistry with the aid of the detector and gas chromatography. The study was carried out in a cold-trapped pumped sodium loop. It was observed that the activity of hydrogen in sodium and in the cover gas is controlled by the temperature of the cold trap. This study also showed that at lower concentrations of hydrogen (∼<2 ppm), the activity of hydrogen in sodium decreases with increasing temperature, which is believed to be due to the interaction between oxygen and hydrogen. However, at higher hydrogen concentrations (∼>2 ppm), the activity of hydrogen is independent of temperature. Finally, it was observed that the permeation of hydrogen dissolved in sodium through a stainless-steel membrane is a diffusion controlled process.