When some calcined dolomite (OCa . OMg) is used in the bed Of a biomass gasifier of fluidized bed type the raw gas produced is cleaner than when only silica sand is used in it as fluidizing medium. In-bed dolomite changes the product distribution at the gasifier exit because of in-situ catalytic reactions promoted by the calcined dolomite. Gasifying with steam-O-2 mixtures causes the tar content in the exit gas to decrease from 12 to 2-3 g tar/m(n)(3), the H-2 content to increase from 25-28 to 43 vol %, and the CO content to decrease from 45 to 27 vol % when the gas and char yields, heating value of the gas, and other main variables also undergo important changes because of the in-bed dolomite. The experimental work here reported is carried out at small pilot plant scale in a 15 cm i.d. atmospheric and bubbling fluidized bed gasifier fed by 10 kg biomass/h. Dolomite is continuously fed to the gasifier, mixed with the biomass in percentages of 2-3 wt % of the total mass flow fed. A 10 wt % of calcined dolomite in the gasifier bed Is enough to significantly improve the product distribution and gas quality.