The effects of mineral matter on the conversion of organic material in low temperature pyrolysis (300-500 degrees C) of the vitrinite-rich Zonguldak bituminous coal and acid-washed coals were investigated. Calcium, iron and magnesium cations were removed from coal with HCl washing. The results indicated that these cations had catalytic effects during pyrolysis. It was also observed that the conversion in the pyrolysis processes was increased with the removal of the illite, kaolinite, chlorite and quartz minerals. The volumetric swelling ratios of the chars obtained in different experimental conditions were measured using the pyridine swelling technique and the extent of crosslinking in the macromolecular network of chars was examined. It was found that the cross-linking increased as the pyrolysis temperature was increased from 300 to 500 degrees C. (C) 2000 Elsevier Science Ltd. All rights reserved.