Biomass is usually gasified above the optimal temperature at the carbon-boundary point, due to the use of different types of gasifiers, gasifying media, clinkering/slagging of bed material, tar cracking, etc. This paper is focused on air gasification of biomass with different moisture at different gasification temperatures. A chemical equilibrium model is developed and analyses are carried out at pressures of 1 and 10 bar with the typical biomass feed represented by CH1.4O0.59N0.0017. At the temperature range 900-1373 K, the increase of moisture in biomass leads to the decrease of efficiencies for the examined processes. The moisture content of biomass may be designated as "optimal" only if the gasification temperature is equal to the carbon-boundary temperature for biomass with that specific moisture content. Compared with the efficiencies based on chemical energy and exergy, biomass feedstock drying with the product gas sensible heat is less beneficial for the efficiency based on total exergy. The gasification process at a given gasification temperature can be improved by the use of dry biomass and by the carbon-boundary temperature approaching the required temperature with the change of gasification pressure or with the addition of heat in the process. (C) 2009 Elsevier Ltd. All rights reserved.