Measurements of the ignition time and surface temperature, at the time of ignition for cellulosic fuels in varying atmospheric environments were made in order to help illuminate the mechanism responsible for the spontaneous ignition of these materials exposed to thermal radiation. It was assumed that the mechanism would fall into one of three categories, and these were (1) A piloted self-ignition, which is dependent in some way upon the temperature of the pyrolyzing solid. (2) A thermal self-ignition which is governed mainly by gaseous heat transfer. (3)) A free-radical chain branching self-ignition which should be quite pressure dependent. The varied environmental atmospheric parameters were: (1) Total atmospheric pressure, holding the oxygen concentration constant; (2) oxygen concentration with the total pressure held constant; and (3) the thermal conductivity of the atmospheric environment by use of different diluent gases with both the total pressure and oxygen concentration held constant. Both the surface temperature and ignition response data provide evidence which indicate the thermal self-ignition to be the most likely mechanism responsible for this phenomena.