Calcined dolomites, limestones, and magnesites are active and inexpensive solids for cleaning raw hot gas from biomass gasifiers with steam. The variations of their activities with time-on-stream are studied here. Simultaneous coke formation and coke elimination by steam gasification increases the life of these "naturally occurring" catalysts under some circumstances. The lives of these solids are studied at different temperatures (800-880 degrees C), space times (0.08-0.32 kg of dolomite . h/nm(3)), particle diameters (1-4 mm), and types of solid. Not much deactivation was observed for tar concentration in the raw gas below 48 g/nm(3), particle diameters of less than 1.9 mm, temperatures above 800 degrees C, and space times above 0.13 kg . h/nm(3). The effectiveness of these calcined minerals is compared with that of an inert material (silica sand) and with a commercial steam reforming catalyst (R-67 from Haldor Topsoe).