Gasification reactivity of high-heating-rate chars (HHR-chars) in steam, carbon dioxide and their mixtures was investigated in a new macro-TG experimental device. The higher reactivity of the HHR-chars was highlighted by a comparison with reference chars prepared at a low heating rate (LHR-chars). It was found that the char reactivity in a mixed atmosphere of steam and carbon dioxide can be expressed as the sum of the individual reactivities obtained in single-atmosphere gasification experiments. This result was not dependent on the pyrolysis heating rate. In addition, gas-alternation gasification experiments - for both HHR-chars and LHR-chars - showed that gasifying the char with CO2 up to 30% of conversion does not affect its reactivity to H2O. Altogether, the results tend to indicate that the two reactant gases H2O and CO2 react on separate active sites when mixed atmospheres are used, and that CO2 does not affect the char structure to favor or inhibit the char-H2O gasification reaction. (C) 2013 Elsevier Ltd. All rights reserved.