A modified Olivine, enriched in iron content (10% Fe/Olivine), and a natural bauxite, were tested in the in-bed reduction of tar and alkali halides (NaCl and KCl) released in a process of biomass steam/O-2 gasification. The tests were carried out at an ICBFB bench scale reactor under the operating conditions of: 855-890 degrees C, atmospheric pressure, 0.5 steam/biomass and 0.33 ER ratios. From the use of the two materials, a reduction in the contaminant contents of the fuel gas produced was found. For the alkali halides, a decrease up to 70%(wt) was observed for the potassium concentration, while for sodium, the reduction was found to be quite poor. For the organic content, compared to unmodified Olivine, the chromatographically determined total tar quantity showed a removal efficiency of 38%(wt). Moreover, regarding the particulate content a rough doubling in the fuel gas revealed a certain brittleness of the new bed material. (C) 2012 Elsevier Ltd. All rights reserved.