The present invention relates to a process for preparing optionally alkyl-substituted 1,4-butanediol by two-stage catalytic hydrogenation in the gas phase of C.sub.4-dicarboxylic acids and/or of derivatives thereof having the following steps: a) introducing a gas stream of a C.sub.4-dicarboxylic acid or of a derivative thereof at from 200 to 300.degree. C. and from 10 to 100 bar into a first reactor or into a first reaction zone of a reactor and catalytically hydrogenating it in the gas phase to a product which contains mainly optionally alkyl-substituted .gamma.-butyrolactone; b) introducing the product stream obtained in this way into a second reactor or into a second reaction zone of a reactor at a temperature of from 140.degree. C. to 260.degree. C. and catalytically hydrogenating it in the gas phase to optionally alkyl-substituted 1,4-butanediol; steps a) and b) being carried out at the same pressure; c) removing the desired product from intermediates, by-products and any unconverted reactants; d) optionally recycling unconverted intermediates into one or both hydrogenation stages, said hydrogenation stages each using a catalyst which comprises .ltoreq.95% by weight, preferably from 5 to 95% by weight, in particular from 10 to 80% by weight, of CuO, and .gtoreq.5% by weight, preferably from 5 to 95% by weight, in particular from 20 to 90% by weight, of an oxidic support, and the product mixture removed from the first hydrogenation stage being introduced without further purification into the second hydrogenation stage.