The upgrading of the raw gas from a biomass gasifier is studied with the commercial steam-reforming BASF G1-25 S nickel-based catalyst. It is located downstream of the gasifier, a bubbling fluidized bed type in which air is used as gasifying agent. To increase the catalyst lifetime; a guard bed of a calcined dolomite at 800-850 degrees C is used. It decreases the throughput of tar entering the catalytic bed to amounts below 2 g tar/m(3)(NC). This work is focused only on the catalytic bed which easily decreases the tar content in the gas to only 1-2 mg/m(3)(NC). Variables studied include the particle diameter of the catalyst, time-on-stream, temperature of the catalytic bed, and gas and tar, compositions, Both tar and gas compositions in the catalytic (Ni) reactor depend on the equivalence and WC ratios existing in the gasifier and on the operating conditions of the guard bed of dolomite. A simple kinetic model is used to describe the overall tar elimination network. Its overall kinetic constant is used as index of the catalyst activity for tar elimination. Values of this overall kinetic constant are given for very different operating conditions.