The basis for defining heats of formation and combustion is the standard state. The standard state is defined as a temperature of 25°C and a pressure of 1 atmosphere (atm). For heats of formation, the standard state is also defined as the pure substance in its most stable form at the standard temperature and pressure. For heats of combustion, the standard state is defined as the complete combustion of the substance to form carbon dioxide and water, with the water formed being in the liquid state.

The heat of formation of a compound is the change in enthalpy that occurs when one mole of the compound is formed from its constituent elements in their standard states. The heat of combustion of a compound is the change in enthalpy that occurs when one mole of the compound is completely combusted to form carbon dioxide and water, with the water formed being in the liquid state.

The heats of formation and combustion of compounds are important thermodynamic properties that can be used to calculate the enthalpy changes of chemical reactions.