Various waste wood species including primary sludges from spruce wood debarking stage, spruce bark, aspen poplar wood and a mixture of spruce wood and aspen bark were pyrolysed under vacuum in a bench scale reactor and a Process Development Unit. The low water content pyrolysis oils were recovered and derivatized by diazomethane to methyl esters. The oils were then fractionated on a silica gel column into three fractions. High molecular weight carboxylic acid methyl esters and resin acid methyl esters were recovered in the second fraction. The methyl ester fractions were analyzed by capillary gas chromatography-mass spectrometry. Fatty acid and resin acid methyl esters in the range of C-4-C-28 were identified and quantified. The acid distribution in the pyrolysis oils was studied regardless of the initial wood sample location, age, and storage time before pyrolysis. Free acid methyl ester and other ester (e.g. criglycerides) contents of the pyrolysis oils were determined The primary sludges and softwood bark were Soxhlet extracted with acetone and their acid contents were determined and compared with the pyrolysis oils. The production of high molecular weight acids under varying pyrolysis conditions is discussed.