Comparison of the adsorption characteristics of the desiccants silica gel, alumina, and molecular sieves has shown that molecular sieves have by far the greatest capacity of the desiccants at the low partial pressures considered. Equilibrium data in the form of isotherms were established over the range of variables expected in the coolant circuit of a proposed Australian H.T.G.C. reactor. The mass transfer from the gas phase to molecular sieves is such that no correlation could be attempted for the adsorption zone height; the height proved to be too small.