The objective of this study was to investigate thermal conversion of a perennial shrub, Euphorbia rigida biomass sample with catalyst in inert (N-2) and steam atmospheres. Experimental studies were conducted in a well swept fixed bed reactor with a heating rate of 7 degrees C/min to a final pyrolysis temperature of 550 degrees C and with a mean particle size of 0.55 mm in order to determine the effect of different atmospheres with various catalyst ratios on pyrolysis yields and characteristics. The catalyst ratios were 5%, 10% and 20% (w/w) under nitrogen atmosphere with flow rates of 50, 100, 200 and 400 cm(3)/min and steam atmosphere with well-swept velocities of 12, 25 and 52 cm(3)/min. The optimum oil yield was obtained as 32.1% at the nitrogen flow rate of 200 cm(3)/min, while it was obtained as 38.6% at steam flow rate of 25 cm(3)/min when a 10% catalyst by weight according to the biomass was used. Higher oil yields were observed when biomass sample was treated in steam atmosphere than in inert (N-2) atmosphere. The oil composition was then analysed by elemental analyses techniques such as IR and GC-MS. The oil products were also fractionated by column chromatography. The bio-oils obtained at both atmospheres contain mainly n-alkanes and alkenes, aromatic compounds; mainly benzene and derivatives and PAHs, nitrogenated compounds and ketones, carboxylic acids, aldehydes, phenols and triterpenoid compounds. More oxygenated compounds and less substituted n-alkanes and alkenes were obtained in catalytic pyrolysis of E. rigida in the steam atmosphere. The experimental and chemical characterisation results showed that the oil obtained from perennial shrub, E. rigida can be used as a potential source of renewable fuel and chemical feedstock. (C) 2007 Elsevier Ltd. All rights reserved.