The effect of variation in fuel composition on exhaust emissions from a single-cylinder engine has been studied with eleven different fuels. Exhaust emissions, at fixed engine speed and load, were measured over a range of air-fuel ratio (A/F) settings. At identical A/F values, CO, HC, and NO emissions varied significantly from fuel to fuel. At the same equivalence ratio (ϕ), CO emissions were identical for all fuels. However, at fixed ϕ values, both NO and HC decreased with increasing fuel hydrogen-to-carbon ratio.