The present invention relates to a system using and/or processing fuel, wherein a catalytically active element is desensitised to impurities present in the fuel. The inventors thus surprisingly found that a fuel processing system, which may be a fuel cell or asub-unitofa fuel cell 5 system, could be successfully protected from impurities, in particular elemental sulfur and compounds containing sulfur. The protective effect is obtained by using an additive, preferably a halogen, in the preparation of the catalytically active element.