Reduction of nitrogen oxides emissions from diesel engines

A process for reducing nitrogen oxides emissions from a diesel engine comprising forming an emulsion of an aqueous solution in diesel fuel which comprises (a) about 5% to about 70% solution by weight of a compound selected from the group consisting of urea, ammonium carbamate, ammonium carbonate, ammonium bicarbonate, ammonium formate, ammonium oxalate, ammonium hydroxide, bipyridyl, cyanuric acid, urea-formaldehyde reaction product, and stable amines including hexamethylenetetramine, and mixtures of these, and (b) a catalyst comprising a composition or complex of a metal selected from the group consisting of cerium, platinum or a platinum group metal, copper, iron or manganese, wherein at least about 70% of the water droplets in the emulsion have a particle size below about 5 microns Sauter mean diameter, and supplying said emulsion to a diesel engine to be combusted therein, whereby combustion of the emulsion leads to a reduction in the nitrogen oxides emissions from the diesel engine when compared with combustion of diesel fuel alone.