Commercial steam reforming (nickel-based) catalysts are used for hot gas cleaning and upgrading in biomass gasification with steam-oxygen mixtures. The gasifier used was an atmospheric and bubbling fluidized bed with an internal diameter of 15 cm and a total height of 3.2 m and was continuously fed with 5-20 kg of biomass/h. Eight different catalysts from four different manufacturers (BASF AG, TOPSOE A/S, ICI, and UCI) have been tested. They were located in a downflow fixed-bed reactor of 4 cm i.d. placed in a slip flow after the gasifier. A guard bed with a calcined dolomite was also used before the catalytic bed to decrease the tar content in the raw gas below the limit of 2 g of tar/mg(n)(3), thus avoiding the catalyst deactivation by coke formation. The main variables studied were the temperature of the catalytic bed and the gas composition in the bed. Effects concerning tar elimination will be reported in part 2 of this work. This paper is mainly devoted to characterization of catalysts and to upgrading of the flue gas. H-2 and CO contents increased by 4-14 and 1-8 vol %, dry basis, respectively. CO2, CH4, and steam contents decreased by 0-14, 87-99, and 2-6 vol %, dry basis, respectively. Other parameters varied in the following ways : the lower heating value decreased by 0.3-1.7 MJ/m(n)(3), gas yield increased by 0.1-0.4 m(n)(3)/kg of biomass daf, and apparent thermal efficiency increased by 1-20%. The results presented allow screening of the best catalysts to get an upgraded and useful gas in biomass gasification with steam-oxygen mixtures.