The paper focuses on the gasification of biomass chars in H2O, CO2 and their mixtures. The first part is dedicated to the study of the influence of particle size and temperature in single atmosphere gasification experiments. The Thiele approach was successfully applied to assess the extent of diffusional limitations. We also studied the mixed atmosphere gasification of 0.2 mm chars at various temperatures and found that the char reactivity is fairly represented by an additive law at relatively low temperatures while it is lower than the sum of the individual contributions for high temperature cases. Similarly, we assessed the effect of particle size for mixed atmosphere experiments and found that an additive law was representative of the experimental reactivities for particle sizes from 0.2 to 13 mm. The present work provides useful and worthy information on the char gasification reactivity in conditions close to practical operating ones encountered in biomass gasifiers. (C) 2015 Elsevier B.V. All rights reserved.