The tar and ammonia decomposition activities of dolomite, nickel catalyst, alumina and SiC were compared in different gas atmospheres. Tests were carried out in a fixed-bed tube reactor at 900°C under 2.0 and 5 MPa pressure. Toluene was used as a tar model compound. The gas atmospheres studied were mixtures of nitrogen carrier, toluene and ammonia with H2, H2O, CO, CO2, CO2 + H2O, H2 + H2O or CO + CO2. A gasification gas mixture containing all the components was also used. The predominant reaction of toluene in the empty tube and over SiC and alumina was hydrocracking. With dolomite and nickel catalyst, steam and CO2 reforming types of reaction took place at high rate, CO2 reforming being the faster. However, in gasification gas the rates of steam and CO2 reforming reactions were lowered due to the inhibiting effect of CO, CO2 and H2. Ammonia reacted readily with CO2 even with the inert reference materials and very rapidly over the nickel catalyst. The presence of toluene increased the ammonia reaction rate on the nickel catalyst. Ammonia decomposed in the gasification gas with the nickel catalyst at the same rate as in H2 + H2O-N2.