Previous studies observed that slow copyrolysis of wood and plastic in enclosed autoclaves produced an upgraded raw bio-oil with increased hydrogen content. We now demonstrate that fast simultaneous pyrolyses of 50:50, w/w, pine wood/waste plastics in a 2 kg/h lab scale auger-fed reactor at 1 atm, with a short vapor residence time, generates higher heating value upgraded bio-oils. Three plastics: polystyrene (PS), high density polyethylene (HDPE) and polypropylene (PP) were individually copyrolyzed with southern yellow pine wood at 525. 450 and 450 degrees C, respectively, to generate modified bio-oils upon condensation. These liquids exhibited higher carbon and hydrogen contents, significantly lower oxygen contents, higher heats of combustion and lower water contents, acid values and viscosities than pine bio-oil. The formation of cross-over wood/plastic reaction products was negligible in the oils. Simultaneous pyrolysis process design requires using a temperature at which the plastic's thermal decomposition kinetics produce vapors rapidly enough to prevent vaporized plastic from condensing on wood chars and exiting the reactor. (C) 2009 Elsevier Ltd. All rights reserved.