NOVELTY - The approaching feedstock and steam pass along a side zone of the catalyst, in which they are not allowed to contact the catalytically-active material. They are introduced into the catalyst (3) at the base (3b), at approximately the highest temperature.    USE - A steam-hydrocarbon reformer for e.g. vehicular use. It is used to produce hydrogen-rich reformate for a fuel cell system used to supply mobile equipment with electrical energy (claimed).    ADVANTAGE - This design avoids overheating of hydrocarbon approaching the catalyst, to minimize undesirable cracking reactions and soot formation. Catalyst overheating is also avoided. In addition, the construction is compact. The unit is very efficient and reliable in all loading conditions, especially when partially-loaded. The raw materials are liquid at room temperature.    DETAILED DESCRIPTION - Steam and feedstock flow through the catalytic region (3), the temperature reducing from maximum. This region is U-shaped, its base (3b) being hottest, and the opposite region (A) being at a lower temperature. The supply passes between the limbs (3a) without contacting the catalyst, to the base (3b). The reformate is removed at the end remote from the base. The heated region (2) of the unit (1) has a casing (8) which is U-shaped in cross section. The catalyst is on an axis parallel to the base and projects through it. Heat is supplied by the (vehicle) exhaust gases (A) from the side remote from the base (8a) of the casing and/or the base openings (8b) carrying off the exhaust gases. There are several catalytic regions in tube bundle form, in the heated region. The arrangement and other variants are further detailed, salient features including annular construction of the catalytic region and the use of porous ceramic foam, a porous knit or a metallic foam to form the catalyst. The porous material has a high emission coefficient. The porosity increases through the catalyst; it varies continuously or in stages. The catalyst is in pieces.    DESCRIPTION OF DRAWING(S) - A schematic cross section of an implementation is presented.    unit (1)    heated region (2)    catalyst (3)    base (3b)    limbs (3a)    supply (5)    casing (8)    base (8a)    base openings (8b)