Single-cylinder engine studies show that severity of the test cycle used for deposit accumulation markedly affects the level of exhaust emissions obtained with stabilized combustion chamber deposits. These studies also show that the relative stabilized emission levels with nonleaded and leaded fuels vary significantly with the aromatic content of the base fuel.An extensive evaluation in three groups of passenger cars operated by their owners in normal service showed no significant difference between the stabilized emission levels obtained with commercial nonleaded and leaded fuels. A dynamometer engine test procedure has been developed that simulates short-trip, city-type operation. The accelerated cooldown procedure allows for rapid accumulation of test mileage. Using this dynamometer procedure, the stabilized deposit emission levels of a commercial leaded fuel and a prototype nonleaded fuel are compared.