A study was conducted to determine whether the addition of di-t-butyl peroxide to unleaded gasoline would extend the lean operating limit of a spark-ignition engine. Tests were conducted with a single-cylinder, Cooperative Fuel Research engine. In these tests, the lean misfire limit equivalence ratio was extended by 6 and 10 percent with blends of 1 and 5 volume percent, respectively, of di-t-butyl peroxide and unleaded gasoline, compared to the value obtained with only unleaded-gasoline. It is hypothesized that the improved lean combustion was caused by the dissociation of di-t-butyl peroxide to tertiary butoxy radicals, which in turn reacted with the unleaded gasoline thus increasing the fuel oxidation rate.