The pyrolysis of general biomass materials is modeled via a superposition of cellulose, hemi-cellulose and lignin kinetics. All three of the primary biomass components are modeled with multi-step kinetics involving both competetive primary pyrolysis and secondary tar decomposition reactions. Only “typical” (untreated) feedstocks are considered at atmospheric pyrolysis pressures. The kinetics scheme is then coupled to the porous particle model of Miller and Bellan (1996) along with appropriate properties and heats of reaction to provide a complete model for the pyrolysis of arbitrary biomass feedstocks and sample sizes. Comparisons with past isothermal and thermogravimetry experiments for a variety of biomass materials under both kinetically controlled and diffusion limited conditions show favorable agreement with the model predictions. In addition, discussions are provided which support the use of competetive char production kinetics over single and successive reaction schems which cannot currently be reconciled with observed pyrolysis behavior.