The effect of particle size on the yield and composition of lignin derived oligomers (also known as pyrolytic lignin (PL)) was studied in a fluidized bed reactor. Milled beech wood particles of sizes between 0.3 and 0.55 and cylinders of 3-14 mm were pyrolyzed at 500 degrees C. The lignin oligomers were isolated from bio-oil and analyzed using several analytical techniques (TG, Py-GC/MS, and H-1 NMR). The yield of PL fraction decreased half when particle size raised from 0.3 to 3 mm. DTG analysis of the PL showed higher loss rates between 200 and 600 degrees C as particle size increased from 0.55 to 3-4 mm. Py-GC/MS results suggested a dramatic decrease of methoxylated phenols (in line with the H-1 NMR studies) and the loss of ether bonds when the particle size was increased from milled small particles of 0.3 mm to cylinders with a diameter between 3 and 5 mm. In general, most chemical changes and changes in PL yield occur between the milled particles (composed mainly of the cell wall) and cylinders with original wood structure. Comparing the milled particles to the cylinders with equal length (equal vapor outflow distance) but varied diameter, the effect on the yield of PL seems to be mainly due to the impact of thermally ejected oligomers on internal cell walls of biomass particles (mass transfer limitations). (C) 2014 Elsevier Ltd. All rights reserved.