Activation of an alumina-supported platinum catalyst by methane treatment was investigated. The treating gas contained 1% CH₄ in N₂ and the treatment temperatures were 573, 673, and 723K. The effluent gas was analyzed during the treatment and the evolution of H₂ and CO₂ was observed. The amounts of chemisorbed H₂ and O₂ were the largest on the catalyst treated at 673K, and they amounted to 65% of the amounts chemisorbed on the hydrogen-treated catalyst. It is considered that at 573K the oxygen in the catalyst was not removed sufficiently and that at 723K a large amount of carbon deposit was built up. Hydrogenation of cyclopropane was carried out at 273K, room temperature, and 328K over both the catalyst treated with methane at 673K and the catalyst treated with hydrogen. The deactivation rate increased with increasing temperature, and the deactivation pattern of the methane-treated catalyst did not appear to be different from that of the hydrogen-treated catalyst.