Interactions of paramagnetic centres in exinite, vitrinite and inertinite from durain, vitrain and clarain of Polish medium-rank coal(85.6 wt% C) with atmospheric oxygen were studied by electron paramagnetic resonance in the X-band (9.3 GHz). The concentrations of paramagnetic centres, the number of component lines in the e.p.r. spectra and their linewidths, g-values and proportions in the total spectrum were determined. Total concentration of paramagnetic centres increases from exinite to inertinite. It decreases after oxygen evacuation from the exinite and vitrinite environment, whereas it increases for inertinite. Four groups of paramagnetic centres exist in these macerals, with e.p.r. lines of different widths: 0.78-0.85 mT (Gauss), 0.48-0.60 mT (Lorentz), 0.34-0.36 mT (Lorentz) and 0.14-0.20 mT (Lorentz). The linewidths of the broad Gauss line increases after oxygen evacuation. Narrowing of the Lorentz lines after evacuation was observed. g-Values were the same in air and in vacuum. Both decrease and increase in concentration of paramagnetic centres of the individual groups after oxygen evacuation were detected. The most active interactions with atmospheric oxygen were observed for paramagnetic centres with the narrowest Lorentz lines existing in all the types of macerals and for paramagnetic centres in exinite and vitrinite with the broadest Lorentz lines.