NOVELTY - Sieve-shaped bodies are coated with catalyst. Catalytic reactor for producing e.g. hydrogen comprises series of catalyst substrates. They are penetrated by small-diameter tubes forming heat exchanger. Fluid or gas to be converted, flows in parallel with medium flowing in tubes and used for indirect heating or cooling. Converted gas is supplied to second reactor. Carbon monoxide is converted to carbon dioxide.    USE - Catalytic reactor for liquids or gases combined with a heat exchanger e.g. for hydrogen production for a vehicle fuel cell, using the water gas shift reaction.    ADVANTAGE - The reactor is small in volume and/or mass, making it more suitable for vehicular use. In this respect the sieve substrate improves on the pellets of prior art. Ceramic implementation is feasible, in addition to the metal version claimed. The configuration promotes turbulence, enhancing mass exchange. Areas available for conversion and heat transfer are large. Greater throughput is to be expected for a given volume. For oxidation reactions the noble metal catalyst is used, whilst for the shift reaction, the copper-based catalyst is preferred. Exothermic and endothermic reactions can be carried out.    DETAILED DESCRIPTION - Sieve-shaped bodies are coated with catalyst. Catalytic reactor for producing e.g. hydrogen comprises a series of catalyst substrates which extend along, and transversely across a channel. They are penetrated by small-diameter tubes forming heat exchanger. Reactor forms part of a vehicular system. Fluid or gas to be converted, flows in parallel with a medium flowing in the tubes and used for indirect heating or cooling. The process is exothermic and the coolant is supplied in parallel co- and counterflow. Converted gas is supplied to a second reactor. Carbon monoxide produced is converted to carbon dioxide by oxygen, with co-flowing coolant supplied in parallel.    An INDEPENDENT CLAIM is included for the method of reactor operation.    DESCRIPTION OF DRAWING(S) - A longitudinal cross section of an implementation is presented.    Channel (1)    Sieve-shaped bodies (2)    Tubes (3)