A new 4 in. (102 mm) diameter fluidized-bed reactor has been built and used to study the effect of biomass particle size on the yield of light gas and tar compounds during gasification. Batch experiments were conducted with white oak milled into four sphere sizes with diameters of 6, 13, 18, and 25 mm, at reactor bed temperatures of 500, 600, 700, 800, and 900 degrees C. As expected, the yields of light gas increase with temperature but then reach a plateau above 800 degrees C. As the particle size increases, a decrease in the formation of light gases, accompanied by an increase in the formation of char, is observed. For larger particle sizes, the commonly used power law dependence of particle devolatilization time with sphere diameter holds true and the transition from heat- and mass-transport control to kinetic control as particle diameter decreases is illustrated. Measurements of the organic compounds present in the syngas are performed using a molecular beam mass spectrometer (MBMS). These measurements indicate that, as the size of the spheres increases, the formation of tars and polycyclic aromatic hydrocarbons (PAHs) increases. These observations further emphasize the important role of heat- and mass-transport restrictions for larger particles in the evolution of products during gasification and its potential implications on gasification process economics.