A method of for treating a hydrocarbon fuel comprises applying a plurality of shock waves to the fuel at a frequency and intensity such as to increase the combustion efficiency of the fuel. An apparatus for treating a hydrocarbon fuel comprises a fuel treatment chamber; an inlet for introducing a hydrocarbon fuel to be treated into the treatment chamber; an outlet for removing a treated hydrocarbon fuel from the treatment chamber; and a means for imparting a plurality of shock waves to fuel within the treatment chamber at a frequency and intensity such as to increase the combustion efficiency of the fuel. The apparatus is particularly suitable for installation in the fuel supply system of an internal combustion engine.