NOVELTY - Heating takes place during cold start-up. In a first phase of operation, the catalytic combustor (11a, 11b, 11c) in thermal contact with the reformer (1) and evaporator (2) is heated above the boiling point of water, by catalytic combustion of hydrocarbon and/or hydrogen with an oxidant gas. In the second phase, water and hydrocarbon are supplied to the evaporator. This mixture has an elevated water/hydrocarbon ratio in comparison with normal operation. The mixture leaving the reformer is supplied via the membrane module (4) to the catalytic burner.    USE - To start up a reformer unit supplying hydrogen to e.g. a vehicular fuel cell.    ADVANTAGE - Operational temperature is reached as quickly as possible from cold start. Hydrogen is produced as early as possible, for use in the fuel cells. Little cost is involved.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) the method of operating a fuel cell system including a cooling circuit which employs the plant described, producing hydrogen from hydrocarbon, and heating up as described, where the waste gas from the catalytic burner is used to heat up the cooling circuit of the fuel cell system; and    (2) a hydrocarbon reformer plant as described.    Preferred features: In the third phase of heating, mixture supply to the catalytic burner is cut back. In the fourth phase of heating, the hydrocarbon fraction in the mixture prepared by the evaporator, is raised as the membrane module temperature rises. During heating, the operational pressure in the membrane module is raised with rising temperature of the heated mixture passing through. The mixture leaving the reformer led via the membrane module is cooled at least during the second phase. This precedes entry into the catalytic burner. Cooling is sufficient to condense out water.    DESCRIPTION OF DRAWING(S) - A block diagram of the processing unit appears.    reformer (1)    evaporator (2)    membrane module (4)    catalytic combustor (11a, 11b, 11c)