AbstractIt is shown that spontaneous combustion is most likely to occur in moist biological materials when they are in equilibrium with a fairly narrow range of relative humidities (R.H.) between 95% and 97%. Experimental results on the adiabatic heating of hay up to ignition temperature confirm this theory. At lower R.H. no appreciable bacterial action takes place, and at higher values, the thermal conductivity increases so rapidly due to water vapour transfer, that the microbiological maximum temperature of about 76° is not exceeded. Chemical reactions that can heat hay to higher temperatures, only occur in the presence of moisture; they are not dependent on previous microbial activity and probably involve direct oxidation of the cellulose. At about 170° the hay has dried out and oxidation of dry hay leads to combustion.