Abstract The dual-fuel method is the easiest and most flexible method for using hydrogen in a diesel engine. In this work, a conventional single cylinder, four-stroke diesel engine has been converted to work on the dual-fuel principle with hydrogen as the inducted fuel. The main objectives of the investigation were to determine the optimum proportions between hydrogen and diesel oil under various engine operating conditions and to study the combustion process in the engine under these conditions. The results show that very satisfactory dual-fuel operation with hydrogen is possible. The thermal efficiencies obtained were comparable with pure diesel operation and up to about half the engine's energy requirement could be derived from hydrogen. The chief problem noticed is that of knocking setting in well before the stoichiometric hydrogen-air ratio. This requires further research for rectification.