The tar-decomposing activities of dolomites and limestones in calcined and carbonated forms were compared. Tests were carried out in a fixed-bed tube reactor at 900°C under 2 MPa total pressure using an N2H2OCO2 gas mixture. Toluene was used as a tar model compound. The calcined rocks decomposed toluene efficiently, but activity was almost totally lost in the carbonated form. This decline of activity took place as soon as the CO2 partial pressure exceeded the equilibrium decomposition pressure of CaCO3. However, even in the carbonated form, the materials decomposed polyaromatics more efficiently than did an inert reference material. With a mixture of the main components of gasifier product gas and in the range 900–1000°C, the tar-decomposing activity was similarly lost under conditions in which the dolomite sample was carbonated.