A transient one-dimensional spontaneous heating model has been formulated to describe the spontaneous heating process at relatively low temperatures. The model consists of three differential equations describing the temperature, oxygen, and moisture concentration in a char pile. These equations have been solved numerically by a finite difference technique, and the influence of the initial char temperature, char reactivity, particle size, and moisture content on the process of spontaneous heating have been examined. Sensitivities are presented for the most important factors affecting the self-heating of coal, including a comparison of coal pile characteristics with and without moisture adsorption, desorption, and migration. This model was also validated against data from large-scale spontaneous heating tests.