Use of carbon black as a stable catalyst for methane decomposition has recently been reported for the CO₂-free hydrogen production. In order to characterize the catalytically active sites on carbon black, acetylene adsorption was examined at 873 K, by using a pulse technique. As the injection was repeated at 873 K a constant amount of C₂H₂ was consumed at each pulse after several injections. A good relationship was obtained between the methane decomposition rate over various carbon blacks at 1123 or 1173 K and the constant acetylene consumption at 873 K. This can be utilized as a probe reaction for comparison of activities of different carbon catalysts. A reasonable model had been proposed for the constant acetylene consumption on the armchair face at the edges of graphene layers. The constant consumption may be explained by the "acetylene-chemisorption-hydrogen-abstraction(ACHA)" mechanism and can also explain the stable activity of carbon blacks. In addition to this, models for the C₂H₂ chemisorption processes on different active sites, such as the zigzag face and corners, were proposed and discussed.