Biomass gasification coupled with in situ CO2 removal using CaO is a promising process for the sustainable production of H-2. Thermodynamic equilibrium modeling predicts biomass conversion to gas containing up to 81.4 vol % H-2 (dry basis). However, in practice, discrepancies between theoretical and experimental results demonstrate the significance of nonequilibrium phenomena. In this context, we have developed a simultaneous thermogravimetric-mass spectrometric (TG-MS) technique to study the influence of: (i) heating rate, (ii) CaO loading, (iii) evolved gas residence time, and (iv) reaction atmosphere. The results of this study give insight into the decomposition mechanisms of biomass in the presence of CaO, including the influence of: (i) CO2 removal, (ii) sorbent hydration (Ca(OH)(2)), (iii) intrinsic heating and swelling effects associated with the exothermic gas-solid absorption reactions, and (iv) the direct catalytic role of CaO for char gasification and tar elimination. (C) 2008 American Institute of Chemical Engineers.