The heat of combustion is more precisely termed the enthalpy of combustion and is given the symbol H_comb. The term heat of combustion is typically used interchangeably with the more precise term the enthalpy of combustion. The enthalpy of combustion of the heat of reaction for the combination of one mole of a substance with oxygen to form oxidized forms of the elements of the substance, such as carbon dioxide from carbon atoms, water from hydrogen atoms and sulfur dioxide from sulfur atoms. As is the case for other types of enthalpy, the heat of combustion is the heat absorbed for one mole of the particular substance that undergoes the reaction (combustion) under three specific conditions: (1) the pressure remains constant, (2) the only possible work that occurs is expansion against the atmosphere (so-called PV work) and (3) the temperature remains constant during the process.

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