The sulphidation of calcined limestone with H2S, COS and mixtures of H2S and COS was studied in a thermogravimetric analyzer at temperatures between 500 and 700°C. The applied H2- and COS-pressures corresponded with those in coal gas produced by modern coal gasifiers, i.e. 50¿12,000 Pa for H2S and 50¿1000 Pa for COS. The reaction orders in H2S and COS as well as the activation energies of the involved reactions were determined. The mechanism of sulphidation was examined by simulating measured conversion vs time behaviour with the grain size distribution model of Heesink et al. (1993, Chem. Engng J. 53, 25¿37).