A process is described for converting and utilizing oxygen-containing polymers to form hydrogen and alkali metal carbonates, in which the polymers are brought into intimate contact with a melt mixture of alkali metal hydroxide and alkali metal carbonate to form hydrogen and alkali metal carbonate, and the alkali metal carbonate formed is removed from the reaction mixture during the reaction and alkali metal hydroxide is optionally metered in. The reaction is preferably carried out under conditions which lie on a point on the Liquidus line of the system. It is possible in accordance with the invention to process natural or synthetic polymers, for example polyester, polyether, wood, etc. It is particularly suitable for processing fiber-reinforced composite materials. Also described is an apparatus for performing the process according to the invention.