Self heating of large coal piles stored for long periods (1-6 months), due to atmospheric oxidation (chemisorption\oxidation) raise a maintenance problem in the storage sites near large power stations which use bitumineous coals as the fuel. Thus hot spots are formed resulting in reduction in calorific value of the coals and in extreme cases eruption of fires. The main gas released during the storage is carbon dioxide but also some carbon monoxide, low molecular weight organic molecules (C 1-5) and water are produced. It has been established that moreover, bituminous coal exposed to mild oxidation conditions (temperature range 40-120°C in air atmosphere) emit also molecular hydrogen as a side product. These reactions have been observed for a variety of bituminous coals worldwide. The amount of molecular hydrogen formed (which is a reduction product) is surprisingly linear to the amount of oxygen (oxidation reagent) consumed by the coal. It has been suggested that the low temperature oxidation produces some surface hydroperoxide groups which do oxidize formaldehyde (which is also released by the self heating of the coal) to yield the ustable cyclic intermediate dioxirane, CH202, which subsequently decomposes to molecular oxygen and carbon dioxide. The mechanism of reaction and the effect of water addition to inhibit molecular hydrogen production will be discussed in detail.