The critical oxidation temperature or the crossing point temperature is often used as a parameter in studying the self heating characteristics of coals. Although the mechanism of auto-oxidation is not fully understood as yet, it is known that the moisture plays a very important role in the reaction. The paper report the influence of moisture in the self heating characteristics as investigated from the changes in the critical oxidation temperature of coals. The investigation also includes the effect of humid air (i.e. at various percentages of relative humidity). It has been observed that the self heating characteristic of coal is influenced by both the sources of moisture i.e. the moisture in coal and the humidity of air. However, the moisture in coal plays a more signification role during spontaneous combustion.