Measurements were taken of the speciated aldehyde and ketone exhaust emissions from a modern four-cylinder engine fuelled with natural gas. The effect on these emissions of varying the engine operating parameters spark timing, exhaust gas recirculation rate, engine speed, and fuel/air equivalence ratio was examined. The influence of these operating parameters on the complete reactivity-weighted emissions with natural gas fuelling is predicted.With stoichiometric fuel/air mixtures, both the total hydrocarbons and formaldehyde emissions declined with increasing exhaust gas temperature and increasing in-cylinder residence time, suggesting that formaldehyde burn-up in the exhaust process largely controls its emissions levels. Closer examination of the aldehyde emissions shows they follow trends more like those of the non-fuel, intermediate hydrocarbon species ethane and acetylene, than like the trends of the fuel components methane and ethane. Aldehydes are also intermediates in the combustion of natural gas.For tests in which the fuel/air equivalence ratio was varied, aldehyde emissions behaved very differently than the hydrocarbon intermediates. This behaviour arises from the increasing importance of methyl-methyl recombination to form ethane, over methyl reaction with oxygen to form formaldehyde, as the mixture is enriched. Carbon emitted in the form of formaldehyde on the lean side of stoichiometric is emitted as the ethane by-products ethane and acetylene on the rich side.