In order to test and quantify the effect of heat of rewetting on the propensities for spontaneous combustion of coals, especially low rank coals, a new experimental procedure and a mathematical model were developed based on the standard R70 adiabatic oxidation method. Dry and moist tests were performed on coal samples collected from Powder River Basin and from South America coal fields. Based on the linear and moving average parts of the self-heating curves, self-heating indices in terms of Moist R70 and Rtes were proposed. The developed analytical model, calibrated with the experimental data, was used to simulate self-heating of coal with different amount of water condensation (2%, 5%, 10% and 20%) and relative humidity (40%, 60% and 90%). It was shown that an increase of water condensation and relative humidity significantly promotes the self-heating rate of coal. A practical consequence of this finding is the heat of rewetting mechanism should not be neglected in any precautionary or preventive measures to avoid potential spontaneous combustion in PRB and Colombia coal mines.