In this study, the combustion of butanol, neat and mixed with gasoline, was investigated on a 0.6 liter two-cylinder spark ignition engine with fully adjustable fuel injection and spark timing, coupled with an eddy current dynamometer. Two isomers of butanol, n-butanol and iso-butanol, were examined. Butanol can be produced from non-food renewable resources and is one of the fuels exploited in the search of energy security and independence and of replacement of fossil fuels. Compared to the traditionally used ethanol, butanol does not exhibit hygroscopic behaviour, is chemically less aggressive and has higher energy density. On other hand, different laminar burning velocity and higher boiling temperature of butanol, compared to gasoline, requires some countermeasures to keep the engine operation reliable and efficient. Optimum spark timing for n-butanol, iso-butanol and their mixtures with gasoline, response to both lean and rich mixture composition, and three-way catalyst performance are reported and discussed for selected steady state operating conditions. For low loads, the results suggest a spark advance decrease, larger for n-butanol than for iso-butanol, while at higher loads, the optimal timing is comparable for all fuels. Deviation from stoichiometric mixture composition did not affect significant points of heat release of and its variability for all tested fuels and fuel mixtures. The three-way catalyst performance is generally comparable when either isomer of butanol in any concentration is used. Furthermore, for some components at certain conditions observed conversion efficiency was increased when butanol is combusted. Overall, the combustion of both n-butanol and iso-butanol and their mixtures with gasoline was, at steady state operation with fully heated engine, and after adjustments of spark timing and of fueling rate, comparable with gasoline. Starting of a cold engine, a known problematic aspect of using any alcohol fuel, was not addressed in this study.