Phenolic bio-oil produced by the direct liquefaction of Eastern white pine (Pinus Strobus L.) sawdust in a hot-compressed ethanol-water (1:1 w/w) medium at 300 degrees C was used to partially substitute for phenol in the synthesis of bio-oil-phenol-formaldehyde (BPF) resol resins. Bio-based resol resins with high levels of phenol substitution (up to 75 wt%) could be used as plywood adhesives because of the low molecular weights found for the phenolic bio-oil (weight-average molecular weight = 1072 g/mol, number-average molecular weight = 342 g/mol). The properties of the BPF resol resins were analyzed by differential scanning calorimetry, Fourier transform infrared spectroscopy, gel permeation chromatography, and thermogravimetric analysis. All of the experimental BPF resins possessed broad molecular weight distributions but had similar chemical/thermal properties compared to a conventional phenolformaldehyde (PF) resol resin reference (or 0 wt % BPF resin). The BPFs exhibited the typical properties of a thermosetting PF resin, for example, an exothermic curing temperature of 140-150 degrees C and an acceptable residual carbon yield of 48-72 wt % nonvolatile content at 700 degrees C. The experimental BPFs were applied as adhesives in the assembly of plywood, and then, the dry/wet tensile strengths were evaluated. The tensile strengths of the dry plywood samples bonded with the BPF resins up to a high ratio value of 75 wt % bio-oil exceeded or were comparable to that of the conventional pure PF resin adhesive. All of the BPF-resin-bonded plywood samples gave wet tensile strengths comparable to those of the conventional PF adhesive. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2743-2751, 2011