A 2D non-isothermal and non-equimolar reaction-diffusion model is developed in order to assess the diffusional effects that may take place during the CO2 gasification of a biomass char contained in the cylindrical crucible of a horizontal-arm thermogravimetric apparatus (TGA). The model takes into account the chemical reaction rate and the effective transport properties as a function of the local conversion and therefore as a function of time and position within the char bed. Intraparticle diffusion and structural changes of the char bed during the gasification are also included. The model results showed good agreement with the experimental ones obtained at different temperatures and CO2 partial pressures, and points out the relevant role of diffusional effects in TGA experiments. The effectiveness factor defined as the ratio of the measured to the intrinsic reaction rates at the same gas conditions stronglly depends on temperature and CO2 partial pressure and may be as low as 0.2 at 950 degrees C and 50% char conversion. The marked variation found for the effectiveness factor at different conversions suggests the necessity of a complementary analysis of the variation of the diffusional effects with conversion when TGA experiments for kinetic determination are carried out. (c) 2005 Elsevier Ltd. All rights reserved.