If the reaction occurs when insufficient oxygen is present, both carbon monoxide (CO) and carbon dioxide (CO₂) will be in the combustion products and the heat released in the reaction will be reduced to Q'. When the fuel/air ratio differs from that required for stoichiometric combustion, the combustion condition can be described in terms of the "Equivalence Ratio."

Equivalence Ratio = (Actual Fuel/Air ratio)/(Stoichiometric Fuel/air ratio) = f

If all of the heat of combustion is converted into internal energy of the reaction products, their temperature will increase to a value known as the "Adiabatic Flame Temperature." In a real flame, heat losses and dissociation reactions will reduce the flame temperature below the adiabatic value.