In the reaction of CO with H2 over a catalyst such as Zn oxide and Ca bichromate, to produce alcohols, the catalyst is kept clean by passage of 1 kg. anthracene and 7 m.3 mixed gas per hr. per kg. of catalyst through the reactor. Similarly, an equal volume of anthracene oil is passed with heavy tar over the molybdic acid, chromic acid, and manganese carbonate catalyst employed for hydrogenation, to give a net charge rate of 1 kg. mixed oil per kg. catalyst per hr. Building up of heavy carbonaceous material upon the catalyst surface with consequent lowering of its activity is thus prevented.