Pyrolysis to produce bio-oil from sewage sludge is a promising way, to not only improve the economical value, but also to reduce pollutants associated with sludge. The aim of this study was to evaluate the production of oil from primary, waste activated and digested sludges. The pyrolysis was performed in a laboratory-scale horizontal batch reactor. The operating temperature ranged from 250 degrees C to 500 degrees C, while a gas phase residence time of 20 min was maintained with 50 ml/min of nitrogen gas as a purge flow. The maximum oil yield was achieved with primary sludge at 500 degrees C. Temperature and volatile solids were the most important factors affecting the yield of oil and char, however, sludge type also affected both results. Pre-treatment of sludge with either acids, a base or a catalyst (zeolite) did not improve the quantity of oil produced. The economic values of the oil produced from primary, TWAS, and digested sludges were estimated as 9.9, 5.6, and 6.9 phi/kg-ds when the value of oil is 32 phi/kg-oil. (C) 2007 Elsevier Ltd. All rights reserved.