Smoldering ground fires can raise mineral soil temperatures above 300°C for several hours with peak temperatures near 600°C. Such temperatures can result in the decomposition of organic material and kill important soil organisms. The heat evolved per unit organic mass was investigated by monitoring the mass of oxygen consumed during smoldering. Organic bulk densities of laboratory samples were comparable to field observations and ranged from 90 to 180 kg m^-3. Moisture and inorganic contents were expressed as mass ratios relative to the organic mass. Moisture ratios ranged from zero to 0.8 and inorganic ratios from near zero (natural peat inorganic ratio) to 4.0. Heat evolved per unit organic mass was independent of organic bulk density and inorganic ratio, and changed little with moisture ratio within the limits of combustion. The average value for all observations (N= 190) was 14.2 MJ kg^-1 with an error less than 4.5%.