The modeling of the conversion of solid fuel in combustion or gasification systems needs a description of the composition of the volatile gases that leave a fuel particle of typical size in a fixed or fluidized bed during devolatilization. Much work has been published on release of volatiles, but a general model of the composition of volatile gases and a comprehensive presentation of the related thermochemical properties of the fuel are still missing. Here, a simplified model is presented whose structure is valid for any solid fuel. The model consists of a heat and mass balance complemented by empirical data. The empirical coefficients have to be specified for certain classes of fuel: the fuels treated in this paper are one type of hardwood and one of softwood, having particle sizes in the range used in utility boilers. The species in the volatile gases are represented by the time mean mass fractions of CO2, CO, H2O, H-2, light hydrocarbons, and heavy hydrocarbons. In addition, a comprehensive set of data is presented for such properties of wood that are needed for modeling of conversion in fixed and fluidized beds. The data are taken from the literature and from measurements carried out in the present work.