NOVELTY - Shortly before transition from first- to second phase operation, fuel-oxidant mixture is dosed with water; and/or fuel is supplied to the multifunction reactor with rising temperature during the first phase; and/or the oxidant mass flowrate is adjusted for sub-stoichiometric conditions.    USE - To assure rapid start up of a hydrocarbon reformer used to supply hydrogen to the fuel cells of a vehicle.    ADVANTAGE - The equipment is brought on line with the most rapid warm-up possible, to minimize duration of the cold start phase, and to get onto the road. Essentially, catalytic combustion of hydrocarbon fuel (or hydrogen) is employed for fast warm-up, the fuel later being reformed with partial oxidation, the multifunction reactor becoming a POX unit (process with partial oxidation of hydrocarbons). Hydrogen produced by reforming is used in fuel cells of the vehicle. The process is further discussed in the text, citing suitable catalysts.    DETAILED DESCRIPTION - Preferred features: Following cold start, the section of the reformer acting as a multifunction reactor (1a), is operated as a feedstock steam reformer and/or as a CO shift reactor. Seconds after start up, transition is made to the second phase. Feedstock to be reformed, or hydrogen, is used for catalytic combustion in the first phase in the multifunction reactor. The remaining part of the reformer (1b), in the second phase, acts at least locally as a post-reformer and CO shift reactor. In this phase, more water is dosed, than in later, warmed-up running. Gaseous products leaving the reformer (1) are supplied to a CO oxidizer (2). This has also been supplied with fuel and oxidant, during the first operational phase, for catalytic combustion. In the second phase a stream containing oxygen is supplied to oxidize the carbon monoxide in the product gas. A catalytic combustor (7) in thermal contact with the reformer is initially fed with fuel and oxidant mixture. It is then fed at least in part, with product gas from the reformer.    DESCRIPTION OF DRAWING(S) - The drawing is a simplified block flow diagram of the unit.    reformer (1)    section of the reformer acting as multifunction reactor (1a)    remainder of reformer (1b)    catalytic combustor in thermal contact with reformer (7)