In the absence of magnetic, nuclear, electrical, surface tension effects one of the two main exergy component is chemical exergy, physical exergy being the other. The objective of this paper is investigating the effects of chemical exergy, one of the least studied exergy component, on exergy efficiency through different methods available in the literature. To achieve this purpose, ideal gas approximation accompanied with chemical exergy-net calorific value expressions and two alternative standard exergy reference environment models were applied to a natural gas combustion chamber. Unlike energy efficiency, exergy efficiency of a combustion process was therefore an array of values rather than a single value depending on the method employed for determining the chemical exergy content of the fuel. Results showed that each method gave a different exergy efficiency value since each had its own assumptions. It was concluded that the method which predicted the closest value to the average can be used for practical uses.