The performance and pollutant emission of a four-stroke spark ignition engine using hydrogen-ethanol blends as fuel have been studied. The tests were performed using 2, 4, 6, 8, 10 and 12 mass% hydrogen-ethanol blends. Gasoline fuel was used as a basis for comparison. The effect of using different blends of hydrogen-ethanol on engine power, specific fuel consumption, CO and NOx emission was studied. Operating test results for a range of compression ratio (CR) and equivalent ratio are presented. The results show that the supplemental hydrogen in the ethanol-air mixture improves the combustion process and hence improves the combustion efficiency, expands the range of combustibility of the ethanol fuel, increases the power, reduces the s.f.c., and reduces toxic emissions. The important improvement of hydrogen addition is to reduce the s.f.c. of ethanol engines. Results were compared to those with gasoline fuel at 7 CR and stoichiometric equivalence ratio. (C) 2003 Elsevier Science Ltd. All rights reserved.