With noble metal catalysts (Pd, Pt, Rh, Ir) present, hydrogen is formed by the interaction of solid calcium oxide with gas mixtures of methane and water vapor, according to CaO + CH4 + 2H2O → CaCO3 + 4H2. Among the metals, Ir and Rh are so active that the reaction takes place at temperatures as low as 600 K. Rate data obtained with these metals show a nearly first order with respect to CH4 pressure, while a negative order with respect to H2O vapor pressure. The apparent activation energies are 171 and 217 kJ/mol for the Ir- and Rh-catalyzed reactions, respectively. On the other hand, Ni does not catalyze the reaction below 733 K, probably due to its strong interaction with H2O vapor.