The strategy of using hydrogen as an additive fuel for the diesel engine to improve exhaust emissions and combustion efficiency has been explored by many researchers in the last decade. The effects of pure hydrogen or the hydrogen-oxygen mixture generated by water electrolysis fueling car or heavy-duty diesel engines were studied, with notably different results. In the present work the supplementary fuel used was the gas produced by the water electrolysis process in a reactor with a special electrode design. Hydrogen-oxygen mixture or pure hydrogen was inducted with air in the engine intake manifold. Performance and emissions characteristics of a 3.6 liters tractor engine, naturally aspirated, were investigated for different operating conditions with gas substitution of diesel fuel up to 12% on energy basis. As expected, significant reductions of smoke, CO and CO₂ up to 25%, 11% and 4% respectively were obtained by increasing the aspiration of gas, while NOx concentrations were higher up to 12%. A slight sacrifice in engine efficiency up to 2% when increasing the gas percentage was also found, which was correlated with the combustion characteristics. Alternative experiments conducted with pure hydrogen as complementary fuel did not result in significant differences in engine performance by comparison with hydrogen-oxygen mixture fuelling.