Abstract Dynamic in situ 9 GHz c.w.-e.p.r. has been used to study the low-temperature oxidation in the interval 20 to 250 °C of hv bituminous coal samples enriched and depleted in exinite, when dried or moisture-saturated. The changes in the e.p.r. spectral parameters and relative total spin concentration are correlated with the maceral composition and moisture content of the samples. These parameters exhibit maxima at ~100 °C. The percentage increase in the spin concentration for dried coal samples between 20 and 100 °C is greater in the exinite-depleted than in the exinite-enriched sample. The 20 °C free radical spin concentration is higher in dried than in moisture-saturated coals but the concentration changes more rapidly with temperature for moisture-saturated samples. This observation is in a good agreement with the interpretation of the lineshape distortion changes determined using radial difference a o d o -Argand diagrams. It is concluded that the moisture content associated with the vitrinite maceral is the most important factor in determining the rate at which the low-temperature oxidation processes occur in coal.