Several common ratios used for determining spontaneous combustion activity rely on comparing the amount of particular products of oxidation with the amount of oxygen consumed to produce these products. Oxidation reactions become more efficient as the coal gets hotter, meaning more products produced for less oxygen consumed. There are many problems associated with accurately determining the true amount of oxygen used for use in these ratios. These problems relate to the monitoring technique (and its associated uncertainties) used to generate the results, deficiencies in the established equations utilised in calculations, dilution with other gases and other sources of oxygen depletion. Typically the oxygen deficiency is over estimated, resulting in the under estimation of the indicating ratio. For this reason the use of “one off” calculations to determine the status of the underground environment is not recommended. Instead trends of these ratios should be used to identify increases which indicate an increase in oxidation intensity. This paper outlines problems associated with the monitoring techniques and equations used to determine the oxygen deficiency.