The work to be described here was undertaken by the author in order to provide accurate data on thermal conduction in gaseous mixtures of deuterium and hydrogen at ordinary temperatures, to be applied to the analysis of such mixtures by the thermal conductivity method developed by A. Farkas and L. Farkas (1934) and others; and also to make an accurate direct determination of the thermal conductivity of pure deuterium. With regard to the latter quantity, the values obtained by other workers to date vary widely, showing a maximum difference of the order of 11%.