Abstract: A process is provided for the hydrogenation of carbon monoxide and/or carbon dioxide to form methane (methanation), using certain sulfur-sensitive nickel-alumina catalysts having a relatively low ratio of nickel surface area to nickel mass. It has been found that when such catalysts become deactivated as a result of sulfur poisoning, they can be reactivated to substantially fresh activity by reduction with hydrogen. Other nickel-alumina methanation catalysts having higher nickel surface areas are found to be essentially non-regenerable by hydrogen reduction following sulfur deactivation.