The rate constant of the CaO-CO2 reaction was studied for two sorbents using an atmospheric thermogravimetric analyzer (ATGA) and a pressurized thermogravimetric analyzer (PTGA). A grain model was used to determine the rate-controlling steps. The intrinsic rate was found to have a variable order with respect to CO2 partial pressure, with a first-order reaction changing to zero-order dependence when the CO2 partial pressure exceeded similar to 10 kPa. No further rate enhancement was observed under pressurized conditions, which was explained by a two-step Langmuir mechanism. The activation energies were 29 +/- 4 and 24 +/- 6 kJ/mol for the limestone and dolomite tested. (C) 2007 Elsevier Ltd. All rights reserved.