The effect of residual chlorine on the metal-catalyzed reaction of solid calcium oxide with a gas mixture of methane and water vapor (CaO + CH4 + 2H(2)O --> CaCO3 + 4H(2)) is investigated at temperatures in the range 613-713 K, using solid samples of Ir/CaO prepared by TPR of Ir/CaCO3 with different chlorine contents below 1.1%. Chlorine-containing samples exhibit anomalous reaction curves characterized by the initial slow and linear progress followed by abrupt acceleration, while chlorine-free samples react more rapidly from the beginning to give simple curves without any acceleration. In both cases, apparent activation energy of 170-190 kJ/mol and reaction order of 0.8-1.0 with respect to methane pressure are determined, demonstrating that the presence of chlorine does not affect the kinetics of the reaction. Thus, the large difference in initial rate is considered to reflect the inhibiting role of residual chlorine. Based on these findings together with the data on surface properties of Ir/CaO, a model is proposed to explain the accelerative progress observed when residual chlorine is present.