In this study, the gasification reactivity of coal chars using CO2/H2O mixtures was investigated and the synergistic effect in reactivity between H2O and CO2 gasification was revealed. First, the coal samples were pyrolyzed under an argon atmosphere at four different temperatures (800-1100 degrees C in 100 degrees C increments). These four different char samples were then gasified isothermally at the four temperatures and five environments ranging from pure CO2 to pure H2O in 20 vol% increments. The results suggest that the addition of CO2 improved the char reactivity, which is higher than the sum of the individual reactivity using either CO2 or H2O below 900 degrees C. At higher temperatures, H2O gasification occurs most rapidly; however, its rate decreased with increasing volume fraction of CO2. The high catalytic activity of Ca species in CO2 gasification was the critical factor for the synergy between CO2 and H2O. (C) 2014 Elsevier Ltd. All rights reserved.