A large-scale apparatus has been used to evaluate the effect of particle size on the spontaneous heating of a coal stockpile. To accomplish this, coal with various ranges of particle size (between 2 and 50 mm in diameter) was oxidized in a cylinder of 3-m length and 0.3-m diameter. A critical range of particle size was observed, below which spontaneous heating leads to flaming combustion. From the results of the oxygen consumption and the temperature of the coal bed, the influence of particle size on the oxidation rate has been analyzed in greater depth. The oxygen conservation equation, in which the oxygen consumption rate is first order in oxygen concentration and obeys the Arrhenius equation, allows the determination of apparent activation energies and preexponential factors. Calculated values of these parameters, by fitting the experimental data to the integrated form of the oxygen conservation equation, have been used to evaluate the effect of particle size. It was of particular interest to determine the operating regime for coal-oxygen reaction and this was achieved by defining an empirical exponent for the particle diameter in the rate equation. This exponent indicated the oxidation regime for a change from chemical to pore diffusion control at successively higher temperatures.