An extension of the concepts advanced by Langmuir regarding the nature of the platinum catalyzed oxidation of hydrogen and the application of the resulting theory to the experimental data observed by Ranschoff and Spiewak for an HRE type recombiner indicates that their data are corrected by the dimensionless equation (see report) equally well, with a mean deviation of 3.8 percent. This expression is recommended as a basis for the design of catalytic recombiners. The catalytic combinations is pictured as consisting of two surface chemical mechanisms, one of which is oxygen diffusion controlled, the other hydrogen diffusion regulated, the mechanism "change-over" occurring at that point in the recombiner where the components are arriving at the catalyst surface by diffusion in stoichiometric proportions. The catalyst volume requirements for three two portions of the bed are shown to be (see report). The hydrogen mole fraction at the mechanism "change-over" point is (see report). And the relationship between the two mass transfer coefficients is (see report). Methods for evaluating the necessary transport properties of the ternary system steam-hydrogen-oxygen for carrying out design calculations are summarized, and the new significant parameters are tabulated and plotted to facilitate these calculations. The question of non-uniform velocity profiles …