The R70 self-heating rate index provides a measure of the intrinsic reactivity to oxygen of organic carbon in coal and shales at temperatures between 40-70C. It has been routinely used for nearly 40 years by the coal industry to rate spontaneous combustion propensity for hazard assessment. Low propensity, and hence low intrinsic reactivity, is assigned to R70 values less than 0.5C/h. Despite this rating, spontaneous combustion events have occurred at mines that fit this low propensity category. Adiabatic oven incubation testing has shown that under normal mine conditions at ambient temperatures, self-heating leading to thermal runaway is not possible for coals or carbonaceous shales with this low intrinsic reactivity. However, there are two possible mechanisms that can alter this outcome. Firstly, if reactive pyrite is present then the oxidation reaction of the pyrite with water and oxygen can act as an initiator of self-heating. This raises the temperature to the point where the carbon oxidation reaction takes over producing thermal runaway to ignition if sufficient carbon is present. Secondly, if an external heat source comes into contact with the coal or carbonaceous shale it can have the same effect of raising the temperature to the point where the carbon oxidation reaction rate is sufficient to sustain self-heating. This paper presents examples of both these mechanisms. CITATION:Theiler, J and Beamish, B B, 2019. Mechanisms for spontaneous combustion events in low intrinsic reactivity coals and carbonaceous shales, in Proceedings Australian Mine Ventilation Conference 2019, pp 160164 (The Australasian Institute of Mining and Metallurgy: Melbourne).