This paper describes experimental results obtained while operating three different diesel engines using fuels treated with a platinum organometallic fuel additive. Platinum group organometallics that are soluble and stable in hydrocarbons were used. The projected cost of the additive is less than the savings associated with the fuel economy improvement measured.The engines generally operated more efficiently and with less emissions when treated fuel was used. Brake-specific fuel consumption was reduced on average by about 5%, and hydrocarbons and carbon monoxide were reduced by up to 46%.Emissions of hydrocarbons and carbon monoxide were reduced from baseline (untreated fuel) in exhaust gas near the engine exhaust manifold. Further reductions were observed after the exhaust gases passed through portions of or the entire exhaust system. Measurement of the platinum metal emitted from the tailpipe indicated that 95% of the platinum introduced in the fuel remains in the engine and exhaust system, mostly the latter.One brief experiment indicated that use of the additive reduced the efficiency loss which occurs when injection timing is retarded to achieve lower emissions of nitrogen oxides. Another experiment indicated that the additive could improve performance of an experimental diesel particulate trap.