A combustion modifier for an aqueous hydrocarbon fuel emulsion that reduces nitrogen oxides (NOx), hydrocarbons, carbon monoxide (CO) and particulate matter from the emissions of internal combustion engines. A class of combustion modifiers used in this invention contain nitrogen. The nitrogen-containing combustion modifiers are nitro compounds, hydroxylamines or salts thereof, nitrogen compounds having at least one strained ring group containing from 3 to 5 ring atoms, nitrites, nitramines and mixtures thereof. The non nitrogen-containing combustion modifier comprises compounds containing at least one strained ring compound.