NOVELTY - A fuel cell system comprises fuel cell stack(s) (1, 5) comprising a first group of membrane electrode assemblies (2) and a second group of membrane electrode assemblies (3) connected in series, and where anodes of the membrane electrode assemblies of the first group comprise an electrocatalyst for the electrochemical oxidation of carbon monoxide.    USE - The invention is used as, e.g. polymer electrolyte membrane fuel cell.    ADVANTAGE - The invention is tolerant to even higher levels of carbon monoxide, such that it is possible to simplify the carbon monoxide clean-up apparatus.    DETAILED DESCRIPTION - A fuel cell system comprises fuel cell stack(s), where the fuel cell stack(s) comprise a first group of membrane electrode assemblies and a second group of membrane electrode assemblies, where the first and second groups of membrane electrode assemblies are connected in series such that a feed stream supplied to the first group is subsequently supplied to the second group, and where the anodes of the membrane electrode assemblies of the first group comprise an electrocatalyst for the electrochemical oxidation of carbon monoxide and are better at reducing the concentration of carbon monoxide in a feed stream comprising carbon monoxide than the membrane electrode assemblies of the second group. An INDEPENDENT CLAIM is also included for a method of operating a fuel cell system, comprising:    (1) supplying a reformate gas (4) stream comprising carbon monoxide to the first group of membrane electrode assemblies, thus providing a reformate gas stream with reduced carbon monoxide concentration; and    (2) supplying the reformate gas stream with reduced carbon monoxide concentration to the second group of membrane electrode assemblies.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of the fuel cell system.    Fuel cell stacks (1, 5)    First group of membrane electrode assemblies (2)    Second group of membrane electrode assemblies (3)    Reformate gas (4)