Sewage sludge was pyrolyzed in a fluidized bed under different operational conditions with the aim of studying the influence of some operational conditions on the composition of the liquid product. These operational conditions were bed temperature (450-650 degrees C), inlet nitrogen rate (nitrogen flow per bed surface unit 0.057-0.090 to s(-1)), and solid feed rate per bed volumetric unit (0.169-0.338 kg s(-1) m(-3)). The composition of the pyrolysis liquids obtained was analyzed by means of GC-MS and GC-FID. Different families of compounds appeared in the liquid samples obtained under the different conditions studied: aliphatic, aromatic, and polycyclic aromatic hydrocarbons; oxygen-containing aliphatic and aromatic compounds; nitrogen-containing aliphatic and aromatic compounds; steroids; chlorine-containing compounds; and sulfur-containing compounds. The composition of the liquids varied qualitatively with the temperature and quantitatively with the three operational parameters studied, with the temperature being the most influential variable. The liquids obtained at 450 degrees C contained oxygen-containing aliphatic compounds > steroids > aliphatic hydrocarbons > nitrogen-containing aliphatic compounds. The compounds found in the liquids obtained at 550 degrees C were quite similar to those obtained at 450 degrees C, although their proportions were different: steroids > aliphatic hydrocarbons > oxygen-containing aliphatic compounds > nitrogen-containing aromatic compounds. The composition of the liquids obtained at 650 degrees C varied considerably, and the most abundant compound groups were nitrogen-containing aromatic compounds > polycyclic aromatic hydrocarbons >= aromatic hydrocarbons.