The influence of oxidation on complex paramagnetic centres system of demineralized coal and coal free of pyrite was compared. Polish orthocoking coal with a carbon content of 87.8 wt% was studied. This coal was oxidized with nitric acid (HNO3), peroxyacetic acid (PAA) and in O-2/Na2CO3 System. Multi-component structure of X-band EPR spectra of the coal samples was numerically analysed. The lineshape and the parameters of the component lines: linewidths and g-factors, were determined. Concentrations of paramagnetic centres were measured. The three groups of paramagnetic centres belonging to different molecular units were found in the studied samples. They were responsible for broad Gauss (DeltaB(pp): 0.49-0.84 mT), broad Lorentz I (DeltaB(pp): 0.18-0.35 mT), and narrow Lorentz 3 (DeltaB(pp): 0.04-0.08 mT) EPR lines. Properties of paramagnetic centres of the simple molecular units with broad Gauss and Lorentz I lines were changed during demineralization of coal and after pyrite removing from coal. g-Values of Gauss and Lorentz I lines increased, Lorentz I lines were broadened and concentrations of paramagnetic centres with Lorentz I lines increased. Oxidation of both demineralized coal and coal free of pyrite with nitric acid led to the highest decrease of the concentration of paramagnetic centres with Gauss lines, narrowing of these lines and increase of g-factor. The higher effects of oxidation on paramagnetic centres responsible for broad lines were observed for coal free of pyrite. Paramagnetic centres with narrow Lorentz 3 lines belonging to multi-ring aromatic units in demineralized coal and coal free of pyrite were not susceptible for oxidation. (C) 2002 Elsevier Science Ltd. All rights reserved.