AbstractWood pyrolysis oil consists of hundreds of complex compounds, many of which are phenolic-based and exhibit hydrophobic properties. Southern yellow pine was impregnated with a pyrolysis oil-based penetrant using both a high pressure and vacuum impregnation systems, with no significant differences in retention levels. Penetrant concentrations ranging from 5-50% pyrolysis oil/methanol on a volume basis were used to determine the threshold concentration for significant physical property improvement. Wood impregnated with penetrant concentration of at least 10% exhibited significant reduction in both moisture sorption and tangential swelling when exposed to a 90% relative humidity and 21°C environment. When exposed to liquid water in a 24-hour soak test, analysis revealed a negative linear relationship between penetrant concentration and both moisture sorption and tangential swell. However, during the course of the 24-hour soak test, a significant linear relationship between penetrant concentration and leachate was determined.