Reactors for flash pyrolysis of biomass are designed to maximize the yield of bio-oil, at the expense of the by-products gas and char. To understand which chemical and physical factors influence the yield to bio-oil, the flash pyrolysis of a cylindrical wood particle with a maximum diameter of 1000 mu m has been simulated by solving the governing equations for mass, enthalpy and momentum conservation for the reactant and products (one dimensional). The flow of vapours is described using the Dusty Gas model [A. Bliek, W.N. Poelje, W.P.M. van Swaaij, F.P.H. van Beckum, AIChE J. 31 (1985) 1666], and the structure of wood is incorporated in the model by applying the random pore model of [N. Wakao, J.M. Smith, Chem. Eng. Sci. 17 (1962) 825]. Typical conversion times for a cylindrical particle increase from 1 to 10 s when the diameter increases from 200 to 1000 mu m at a surface temperature of 823 K. The bio-oil yield (approximately 77%) is hardly affected by the particle size (200-1000 mu m diameter). Obviously tar cracking inside the particle does not occur for the simulated conditions. The heating of a particle is notably delayed by the outflow of vapours. While assuming that they leave the particle in a direction perpendicular instead of parallel to the heat flux, the simulated conversion times appear to decrease with sometimes more than 50%. Finally, the sign and size of the pyrolysis reaction heat is shown to have a distinct effect on the calculated particle conversion time. As an overall conclusion, the results of this work show that an extensive description of internal mass transport phenomena in flash-pyrolysis modelling is not necessary, while accurate knowledge of the reaction kinetics and heat transfer parameters is crucial.