A critical analysis of various genetic and physical factors associated with certain Early and Late Permian and Oligocene coal seams and early Eocene lignite seams of India revealed that the high secondary porosity and small particle size, irrespective of petrographic and rank properties in sub-humid climate, favour spontaneous combustion. Large particle size, high rates of gas emission and highly wet or dry seams, in combination with each other tend to inhibit spontaneous heating in sub-humid, per-humid and semi-arid conditions. The susceptibility to auto-ignition of a coal or lignite seam is not uniform throughout its lateral extent and a high inherent porosity at any given rank stage and a high content of susceptible or oxidizable constituents are not sufficient to cause auto-ignition. However, coal seams having high contents of exceedingly porous inertinite macerals become prone to spontaneous combustion in sub-humid climate even without secondary porosity. A scheme for indexing spontaneous combustion susceptibility, based on the present study, has been proposed which is particularly suited to the Indian context.