Mass loss data obtained in gasification and combustion tests in a thermogravimetric analyzer were used to adjust parameters in an intrinsic chemical reactivity model developed to predict char conversion rates and off-gas compositions when pulverized coal and biomass char particles are exposed to reactive gases. Char reactivity tests were performed in H2O/H-2/N-2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to H2O, in CO2/CO/N-2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to CO2, and in O-2/N-2 environments in order to obtain data to determine kinetic parameters for char intrinsic reactivity to O-2. Wyodak coal and corn stover chars produced at high heating rates were used in the reactivity tests, which were performed under kinetics-controlled conditions. Values determined for the heats of formation and absolute entropies of species adsorbed onto the carbonaceous surfaces as well as the values determined for activation energies of reaction rate coefficients are in the expected ranges for the type reaction considered. The reaction mechanism correctly describes the impact of temperature, H2O, CO2 and O-2 mole fractions, and total pressure on the conversion rates of coal and biomass chars and it captures the inhibiting effects of H-2 and CO on char reactivities to H2O and CO2 over the ranges of temperature, pressure and gas composition relevant to coal and biomass gasifiers and combustors. The heterogeneous reaction mechanism developed accurately predicts the effects of heterogeneous reaction in gasification and combustion environments, including oxy-combustion environments. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.