The effect of fuel anti-knock compounds and combustion chamber deposits on exhaust hydrocarbon emissions was investigated. Six Ford Galaxies equipped with production non-Thermactor 289-CID, 2V engines were operated on a light-duty driving schedule for periods of 12–30,000 miles at Ford’s Michigan Proving Grounds. Three fuel blends were used in mileage accumulation—Indolene Clear (a full boiling range nonleaded gasoline), Indolene 30 (Indolene Clear + 3.0 ml/gallon of motor mix blend), and Indolene 30 + 0.2 theory of an organic phosphorous compound. Two engine lubricants were evaluated for the first 12,000 miles — a petroleum base SAE-10W-30 used for Ford factory fill and a synthetic oil, di-2-ethyl hexal sebacate. After 12,000 miles, all test engines were operated on the petroleum base lubricant.Exhaust emissions were monitored at 3,000-mile intervals on the chassis dynamometer using the CMVPCB* seven-mode procedure. Additional seven-mode cycles were conducted on each engine after combustion chamber deposits were removed at the completion of the mileage accumulation phase.The increase in exhaust hydrocarbon (HC) emissions of the vehicles operating on Indolene 30 and Indolene 30 + 0.2T phosphorous was substantially higher than the HC increase of vehicles operated on Indolene Clear (an average HC increase of 15 ppm for Indolene Clear versus an average HC increase of 171 ppm for Indolene 30).Operation on nonleaded fuel showed no deleterious effect on intake or exhaust valves or other engine components.