Experimental investigation on pollutant emissions and performance of SI engine fueled with gasoline and iso-butanol-gasoline blends is carried out. Engine was operated at speed range of 2600-3400 r/min for each blend (3, 7 and 10 vol.% iso-butanol) and neat gasoline. Results declare that the CO and UHC emissions of neat gasoline are higher than those of the blended fuels for speeds less than or equal to 2900 r/min; however, for speeds higher than 2900 r/min, we have an opposite impact where the blended fuels produce higher level of CO and UHC emissions than the gasoline fuel. The CO2 emission at using iso-butanol-gasoline blends is always lower than the neat gasoline at all speeds by up to 43%. The engine performance results demonstrate that using iso-butanol-gasoline blends in SI engine without any engine tuning lead to a drop in engine performance within all speed range. Without modifying the engine system, overall fuel combustion of iso-butanol-gasoline blends was quasi-complete. However, when engine system is optimized for blended fuels, iso-butanol has significant oxygen content and that can lead to a leaner combustion, which improves the completeness of combustion and therefore high performance and less emissions would be obtained. Finally, the performance and emissions of iso-butanol-gasoline blends are compared with those of n-butanol-gasoline blends at similar blended rates and engine working conditions. Such comparison is directed to evaluate the combustion dissimilarity of the two butanol isomers and also to emphasize which isomer is a superior fuel for SI engines. (C) 2015 Elsevier Ltd. All rights reserved.