This paper presents an investigation on fuel consumption of a diesel engine fuelled with blends of natural gas-diesel and hydrogen-diesel. Experiments have been carried out in a 50 kW, four-stroke, diesel engine. Natural gas and hydrogen have both been injected in the intake manifold, while diesel was directly injected in the combustion chamber. Blends of 25%, 50% and 75% of natural gas in diesel were tested, while the concentrations of hydrogen were 20%, 30% and 50%. No alteration in the diesel injection system has been made when the gaseous fuels were used. The results show that diesel consumption is reduced proportionally to the necessary air amount for stoichiometric burn of the gaseous fuels.