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 2 to 60 bar into a first reactor and catalytically hydrogenating it to a product which contains mainly optionally alkyl-substituted .gamma.-butyrolactone; b) converting the product stream into the liquid phase; c) introducing the product stream obtained in this way into a second reactor at a temperature of from 100.degree. C. to 240.degree. C. and a pressure of from 20 to 250 bar and catalytically hydrogenating it in the liquid phase to optionally alkyl-substituted 1,4-butanediol; d) removing the desired product from by-products and any unconverted reactants; e) optionally recycling unconverted intermediates into one or both hydrogenation stages, said hydrogenation stages both 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 a support, said second reactor having a higher pressure than said first reactor, and the product mixture removed from said first reactor being introduced without further purification into said second reactor.