Because the diesel engine is becoming an increasingly attractive power plant for use in underground mines, concern arises about exhaust emission levels in this environment. The effects of fuel properties and composition on emissions of particulate matter, oxides of nitrogen, unburned hydrocarbons, and carbon monoxide were investigated in this study. Fuels and fuel blends evaluated included DF-1, DF-2, and JP-7; a solution and an emulsion of ethanol with DF-2; emulsions and fumigation of water with DF-2; and LPG with DF-2 pilot injection. All tests were performed on a diesel engine (Caterpillar 3304) representative of underground mining equipment. A micro-emulsion of water in DF-2 proved to be the most attractive fuel based upon availability, reduction of particulates and oxides of nitrogen, and inherent fire-safe characteristics.