6 pages, 7 figures, 5 tables.-- Printed version published on Nov 3, 2008. The aim of the present work is to discuss the differences in the compositions of primary tars collected by means of Gray-King (GK) pyrolysis tests from coking coals of different origin and rank. Ten bituminous coals were pyrolyzed at two different temperatures, 450 and 550°C, which are known to be important in the conversion processes of coal. Low temperature tars were characterized by Fourier Transform Infrared Spectroscopy (FTIR), Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS), and High-Performance Liquid Chromatography (HPLC). The results indicate that low temperature tars are mainly formed by aromatic compounds containing different types of substituents (alkyl, aryl, heteroatomic and heterocyclic). When the coal rank increases, the amount of aliphatic and polar compounds decreases due to the influence of coal composition, while the amount of aromatic compounds such as naphthalenes, phenanthrenes, fluorenes, benzofluorenes, and their alkyl derivatives rises as a consequence of the increase in the aromaticity of the parent coal. The authors thank the European Coal and Steel Community — ECSC — (project 7220-PR/069) and Ministerio de Ciencia y Tecnología — MCYT — (project PPQ2001-1450) for financial support. Peer reviewed