Abstract: The efficiency of fossil fuel combustion equipment, such as engines, furnaces, boilers, and the like, is improved by introducing organic reactive intermediates to the air-fuel mixture prior to ignition. The presence of the reactive intermediates with the fuel improves the combustion reaction by an apparent alteration of the molecular structure of the fuel, resulting in a very high percentage of burning of the fuel. In one embodiment the reactive intermediates are generated from a suitable source compound, such as acetone, which is exposed to a source of ultraviolet radiation thereby converting the acetone into a short-lived, free radical. The short-lived, free radical is then converted into a longer-lived radical by combining it with heavier hydrocarbon molecules. The secondary free radicals are introduced to the air-fuel mixture in advance of the ignition point to improve the combustion reaction.