Natural dolomite powders have been decomposed in high CO2 partial pressure regime with H2O (g) flowing and in the temperature range 913-973 K. All kinetics and microstructure data concerning the first half decomposition step have been compared with equivalent data obtained from the same dolomite powders decomposed under the same conditions with no H2O fluxing. The water vapor reduced the value of the total apparent enthalpy of the reaction from 440 +/- 10 to 345 +/- 10 U mol(-1), but the rate-limiting step of the half decomposition process is not changed, consisting in each case in the transport of the CO2 from the reacting interface. It has been showed that this possibility is plausible because H2O (g) can enhance the sintering of the formed MgO crystallites with subsequent changes of the CO2 mode of adsorption on the MgO surfaces from a strong bonded regime to a weaker one. In presence of H2O (g) and high CO2 (g) pressure, the rate-limiting step of the first half decomposition of dolomite is still the transport Of CO2 across the reacting interface, as it has been proved for the decompositions in CO2 environment. In the temperature range explored, H2O (g) does not change the nature of the solid products which are formed by MgO, CaCO3 and a solid solution of MgO into CaCO3. The microstructure of the solid products is still formed through a shear-transformation mechanism, but H2O (g) enhances the rate at which this step is occurring. Critical analysis of the microstructure data, allow to conclude that the stress level inside the decomposing particles is increasing and enhancing the cracking rate because H2O (g) can increase the MgO grain growth rate. These findings might explain the technical procedure used to decompose the dolomite stone in the ancient ovens. (C) 2003 Elsevier Science B.V. All rights reserved.