The design of an efficient biomass gasifier is essential for the development and practical realization of biomass-to-fuels processes. The study of pine charcoal steam/oxygen gasification in a top-lit downdraft moving-hearth gasifier is presented in this paper. A thermodynamic analysis of the charcoal (C)-H2O-O-2 system was conducted and the composition of the product gas, its lower heating value and the gasifier reduction zone temperature were predicted based on a number of assumptions. A series of the gasifier operations where oxygen flow rate and [H2O]/[O-2] molar ratio were process variables was conducted. It was found that in the range of O-2 flow rates tested (1-5 g/min), higher flow rates of oxygen resulted in higher concentrations of H-2 and CO as well as higher charcoal gasification rates. Likewise, increasing the [H2O]/[O-2] ratio resulted in higher H-2/CO ratio in the product gas as well as higher charcoal gasification rate. The minimum and maximum values of the gasifier hearth loads were estimated. The experimental results obtained underscored the importance of maintaining efficient heat transfer within the gasifier and reducing heat loses to the surroundings. (C) 2013 Elsevier Ltd. All rights reserved.