This paper examines the effects of intraparticle heat and mass transfer on the devolatilization of millimeter-sized biomass particles under conditions similar to those found in commercial coal-fired boilers. A computational model is presented that accounts for intraparticle heat and mass transfer by diffusion and advection during particle heating, drying, and devolatilization. To evaluate the model, devolatilization experiments under high-temperature and high-heating rate conditions were conducted using the Multifuel Combustor at Sandia National Laboratories. Measurements of mass-loss and changes in particle size for millimeter-sized alfalfa and wood particles are presented as a function of reactor residence time. For millimeter-sized particles, both fuels completely devolatilized in approximately 1 s with rapid initial mass loss. The total volatile yield of the wood was 92% on a dry, ash-free basis, significantly higher than that reported by a standard ASTM test, indicating dependence of the ultimate yield on local conditions. Particles for both fuels shrink significantly and become less dense during devolatilization. The comprehensive model accurately predicts the devolatilization behavior of millimeter-sized biomass particles; these measurements could not be reproduced with a simple lumped model that ignores intraparticle transport effects. The comprehensive model is used to examine the effects of particle size and moisture content on devolatilization under conditions representative of those found in coal boilers. Biomass particles of radii up to 2 mm and moisture content up to 50% are considered. As expected, intraparticle heat and mass effects are more significant for larger particles. These effects can significantly delay particle heating and devolatilization; for example, intraparticle effects delay the heating and devolatilization of millimeter-size particles by as much as several seconds for a particle with a 1.5-mm radius compared to predictions of a lumped model. This delay is significant considering the short residence times of commercial boilers and should be accounted for in computational models used to evaluate the effects of biomass-coal cofiring on boiler performance.