NOVELTY - The process involves contacting a feed mixture and oxygen containing gas with a catalyst system (1) provided in a reactor. The upstream part (3) of the catalyst system partly fills the cross-sectional area of the fluid flow path of the upstream part (5) of the reactor and the downstream part (2) of the catalyst system completely fills the area of fluid flow path of the downstream part (6) of the reactor.    USE - For fuel cell system used for fuel cell powered vehicles (claimed).    ADVANTAGE - The degree of feedstock conversion and selectivity towards carbon monoxide and hydrogen of the downstream part of the catalyst system are high. Pressure drop in the upstream part of the catalyst system is reduced. Direct contact between the catalytically active material and metals of the metal carrier, is eliminated. The catalyst carrier and the reactor are resistance to thermal shocks.    DETAILED DESCRIPTION - The process involves contacting a feed mixture comprising the hydrocarbonaceous feedstock and a molecular-oxygen containing gas with a catalyst system having an upstream part and a downstream part. The downstream part is in the form of a porous catalyst bed. The catalyst system is retained in a reactor comprising an upstream part that contains the upstream part of the catalyst system and a downstream part that contains the downstream part of the catalyst system. The upstream part of the catalyst system partly fills the cross-sectional area of the fluid flow path of the upstream part of the reactor and the downstream part of the catalyst system completely fills the cross-sectional area of the fluid flow path of the downstream part of the reactor.    INDEPENDENT CLAIMS are included for:    (1) a reactor comprising a catalytic reaction zone;    (2) a fuel cell system comprising the reactor;    (3) a vehicle provided with a fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows the longitudinal section of a portion of a reactor containing the catalyst system.    Catalyst system (1)    Downstream and upstream part of catalyst system (2, 3)    Upstream and downstream part of reactor (5, 6)