In order to investigate the synergistic effect of high dense polyethylene (HOPE) addition to potato skin (pS) during co-pyrolysis, pure HDPE was pyrolysed as a model compound for determination of the optimum process conditions. Thermal decomposition of pure HDPE was performed at constant heating rate of 5 degrees C min(-1) under various pyrolysis temperatures (400, 420, 450, 470, 500 and 550 degrees C) and sweeping gas flow rates (100, 200, 400 and 800 cm(3) min(-1)). The maximum bio-oil yield was 56% at 500 degrees C under sweeping flow rate of 400 cm(3) min(-1). Thera co-pyrolysis of pure HDPE-pS and waste HDPE-pS mixtures were pyrolysed with various proportions such as 1:0, 1:1, 1:2.2:1 and 0:1. The yield of liquids produced during co-pyrolysis enhanced with increasing weight ratio of HDPE in mixtures. Obtained bio-oils were analyzed in detail with various spectroscopic and chromatographic methods. Co-pyrolysis oil had higher carbon and hydrogen contents, lower oxygen contents with higher heating value than those of pyrolysis. It was concluded that addition of HDPE improved the liquid yields in terms of both quality and quantity. (C) 2012 Elsevier B.V. All rights reserved.