NOVELTY - A membrane reactor for the production of highly pure hydrogen (H2) from a hydrocarbon stream and steam, which has a H2-permeable diffusion membrane for separating H2 from residual gas and a heater, the heater is a hot conductor in the center of the reactor.    USE - The reactor is preferably used in vehicles driven by fuel cells and in domestic heating.    ADVANTAGE - H2 is usually produced from hydrocarbons in 2 stages (steam reforming and shift reaction) but the energy efficiency is less than 75% and the H2 must be purified and concentrated. In existing membrane reactors, the heat for endothermal reforming is normally supplied by burning part of the hydrocarbon feed with air, which results in dilution of the H2 with nitrogen and carbon dioxide. The present process provides the required process heat without burning part of the hydrocarbons in the reactor and makes it possible to produce pure H2. The pretreatment reactor, in which the n-paraffins are produced, can be operated without heating and costly regulation, which are important advantages for mobile operation, making the equipment much simpler than usual. It can also produce a mixture that cannot cause overheating or even destruction of the catalyst, as the adiabatic end temperature can be regulated exactly by the composition of the hydrocarbon mixture.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (A) a process for producing highly pure H2 from a hydrocarbon stream and steam by:    (i) carrying out a steam reforming reaction;    (ii) separating the resultant H2 using a diffusion membrane; and    (iii) carrying out a pretreatment stage for hydrogenation of the hydrocarbon stream, with simultaneous production of n-paraffins, in which the hydrocarbon mixture used contains such a fraction of hydrogenatable hydrocarbons that the resultant heat of hydrogenation is sufficient for operating the pretreatment stage and the pretreated hydrocarbon stream is brought to the desired temperature; and    (B) hydrocarbon mixtures suitable for carrying out this pretreatment stage.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow scheme for the process.    Hydrocarbons and derivatives (1)    Hydrogen (2, 6)    Chemical vaporizer (3)    Membrane reactor (4)    Water (5)    Retentate separation (7)    Fuel gas (8)    Carbon dioxide and water (9)