A general model of the pyrolysis of a wood slab is presented and validated with a set of heat release data. The model is applied to particle half-thicknesses from 5 mum to 5 cm, temperatures from 800 to 2000 K, and moisture contents from 0% to 30%. Internal temperatures, pyrolysis rates and yields of tar, hydrocarbons and char are presented. Four pyrolysis regimes are identified, depending on external temperature and particle size: thermally thin-kinetically limited, thermally thin-heat transfer limited, thermally thick, and thermal wave regimes.