Description : Pulverised New Zealand coal samples have been tested from an initial temperature of 40oC and reacted adiabatically in an oven with oxygen to provide a full temperature history of auto-oxidation up to the self-sustained process of combustion. . This procedure produces a self-heating rate index, R70 , calculated as the ratio of the time taken to reach 70oC (oC/h) . The R70 index is a measure of the coal’s propensity to spontaneous combustion. R70 values for New Zealand coals are much higher than any previously published results. They show a rank dependence, whereby subbituminous coals have the highest propensity to spontaneous combustion (14.91–17.23 oC/h) . A lignite sample has an R70 value of 7.76 oC/h, and 70 high-volatile bituminous B coals have R70 values of (0.31–2.23 oC/h). Samples stored for 2 years show the same rank trend. 70 The nature of this trend is most likely a function of the internal surface area of the coal that governs the available sites for oxidation. Calculating the Suggate rank for any New Zealand coal can be used to rate its propensity to spontaneous combustion. Resin bodies in the subbituminous coal show no propensity to spontaneous combustion.