The acceleration of coal self-heating has long been attributed to the presence of reactive pyrite. However, a definitive means of quantifying this effect has been lacking, particularly from the low ambient temperatures experienced at mine sites. A recently developed moist coal adiabatic oven test has been used to investigate the influence of reactive pyrite on self-heating of a high volatile bituminous coal containing sulphur concentrations from 0.62% to 17.95%. A relationship exists between the amount of pyritic sulphur in the coal and the time taken to reach thermal runaway. However, simply measuring the pyritic sulphur concentration of a coal is not sufficient to quantify the accelerated self-heating effect, as it is the form of the pyrite that determines the pyrite reactivity. These findings will be expanded on in the paper as they have a major significance for the risk assessment of coal self-heating.