The Flame Ionization Detector (FID) used to measure hydrocarbon content in emission samples uses a hydrogen flame that produces little ionization. Hydrocarbons introduced into this flame produce large numbers of ions with ionization proportional to the number of carbon atoms present. This proportionality can be skewed by variations in oxygen content. Oxygen variation in emission samples, cylinders of air or span/calibration gas, and zero air systems are investigated and their effects on emission results are discussed. The oxygen content of the gas under analysis will affect the hydrocarbon concentration reported by the FID. In the example examined in this paper, the oxygen effect was shown to decrease the FTP (Federal Test Procedure) weighted NMHC (Non-Methane Hydrocarbon) results by as much as 7 % for a BMD (Bag Mini-Diluter) sample and 13% for a CVS (Constant Volume Sampling) sample. The effects of oxygen content from zero gas, span gas and calibration gas are much smaller than the effect from a bag sample. The oxygen content effect is greater than the hydrocarbon concentration effect in the concentration region where SULEV (Super Ultra Low Emission Vehicle) exhaust samples are measured.