Wood powder-high density polyethylene (WPE) and wood powder-polypropylene (WPP) composites were pyrolyzed at 550 degrees C in the presence of HZSM-5 catalysts using analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Immediately passing the pyrolysis vapors through the HZSM-5 changed the product distribution by producing aromatic hydrocarbons and eliminating tar formation. Zeolite HZSM-5 was employed with three different silica-to-alumina ratios (25, 50, 260). The influence of catalysts on the yields of aliphatic and aromatic hydrocarbons, furan derivatives, lignin-derived compounds and acetic acid was studied. High yields of aliphatic hydrocarbons formed in WPE or WPP pyrolysis alone. The highest yields of aromatic hydrocarbons from WPE or WPP pyrolysis vapors over HZSM-5 occurred with a zeolite framework Si/Al ratio of 25 (more acid sites), suggesting that the concentration of acid sites inside the zeolite was critical for maximizing aromatic yield. Exposing vapors to HZSM-5 increased the hydrocarbon yields and reduced the amount of acetic acid produced, resulting in increased calorific value. The yields of typical aromatics from catalytic pyrolysis of WPP mixture and composites were higher than those of the calculated values of poplar wood and PP catalytic pyrolysis individually, indicating that a synergistic effect occurred to enhancing aromatic production. Compared to poplar wood/PP mixture, catalytic pyrolysis of the WPP composite generated more multi-ring aromatics and less mono-ring aromatics due to the more complete and thorough mixing achieved by the melt-extrusion process. A general mechanism of catalytic fast pyrolysis of wood-plastic composites was proposed in the presence of HZSM-5.(C) 2016 Elsevier Ltd. All rights reserved.