The selective diffusion of hydrogen in palladium has been employed to determine the presence of hydrogen qualitatively and quantitatively in different gas mixtures. It was found possible to determine concentrations of hydrogen from 100% to 0.001%, or about ten parts per million, in nitrogen, with ease. Also it was found possible to detect the presence of hydrogen at a partial pressure of 10−7mm Hg. This would indicate that it is possible to determine the presence of hydrogen in concentrations of one part in 1010in a gas at atmospheric pressure. There is reason to believe that this technique could be extended even further perhaps to one part in 1014with proper care and presently existing high vacuum techniques.