Differences between primary quinoline insoluble (QI) material of coal tars and their derived pitches caused by preheating carbonization have been studied using microscopic techniques, Coulter Laser particle size analysis and solid-slate C-13 NMR. The selected tars were produced from the same coal using wet and preheated charging at the INCAR experimental coking test plant, which operates on a semi-industrial scale. Specific operational conditions were successfully applied not only to reduce the ash content and the QI and toluene insoluble (TI) contents in the tars produced by the preheating process, but also to modify the nature of the QI particles. In addition. structural changes in the insoluble materials formed during the pitch production from the tar have been monitored.