This paper presents the results obtained of a compression ignition engine (modified to run on spark ignition mode) fuelled with hydrogen-ethanol dual fuel combination with different percentage substitutions of hydrogen (0-80% by volume with an increment of 20%) under variable compression ratio conditions (i.e. 7:1, 9:1 and 11:1) by varying the spark ignition timing at a constant speed of 1500 rpm. The various engine performance parameters studied were brake specific fuel consumption, brake mean effective pressure and brake thermal efficiency. It was found from the present study that for specific ignition timing the brake mean effective pressure and the brake thermal efficiency increases with the increase of hydrogen fraction in ethanol and all hydrogen substitutions showed the maximum increase in brake thermal efficiency and reduction in brake specific fuel consumption value at around 25 degrees CA advanced ignition timing. The best operating conditions were obtained at a compression ratio of 11:1 and the optimum fuel combination was found to be 60-80% hydrogen substitution to ethanol. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.