A laboratory investigation into the effect of inorganic matter on spontaneous combustion behaviour of a Victorian brown coal. Fourteen samples were prepared, namely, the raw coal, water-washed coal, acid-washed coal, and acid-washed coal doped with 11 additives. Each of the samples was then tested in an isothermal reactor to obtain its critical ambient temperature, above which spontaneous combustion occurs. The relative effectiveness of the additives was determined by comparing their critical ambient temperatures with those of the raw coal and the acid-washed coal. Potassium chloride, Montan powder, and sodium chloride were found to be the most effective inhibitors, followed by magnesium acetate, and calcium chloride. The presence of sodium nitrate and ammonium chloride in the coal samples did not show any significant influence on the spontaneous combustion. However, calcium carbonate, sodium acetate, potassium acetate, and pyrite promoted the spontaneous combustion. The effect of additive loading was also investigated for an inhibition agent (KCl) and a promotion agent (NaAc). It was revealed that the effectiveness of these promotion and inhibition agents was enhanced with an increase in the additive loading. Low-temperature oxidation kinetics were also estimated by an energy balance approach and compared with the self-heating potential of these samples. The effects of reactor size and reactor specific surface area on the critical ambient temperatures are also discussed.