The fast pyrolysis mechanism of xylose, xylobiose and xylan was investigated with combined fast pyrolysis experiments and quantum chemistry calculations. The results indicate that acyclic D-xylose is an important intermediate for the formation of the major pyrolytic products in xylose pyrolysis, i.e., 1,4-anhydro-D-xylopyranose (ADX), furfural (FF) and hydroxyacetaldehyde (HAA). ADX derives from the successive cyclization of D-xylose which is the only source for ADX formation. 3-Deoxy-xylose is the most important intermediate for FF formation. HAA results from the C-C bond scission by retro-aldol and cyclic Grob fragment. In addition, the five-membered intermediate derived from the ring-contraction reaction can transform into FF with high selectivity. The dehydrated xylose is also another source for HAA formation. The fast pyrolysis mechanism of xylobiose is similar to that of xylose, and thus similar pyrolytic product distribution can be obtained from them. However, the pyrolytic products of xylan differ significantly from those of xylose and xylobiose, which is ascribed to its high degree of polymerization and the branches in it. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.