Abstract: A gaseous fuel injection system (10) for delivering metered amounts of gaseous fuel into the combustion chamber of an engine, and also a method for injecting gaseous fuel directly into a combustion chamber of an internal combustion engine. The gaseous fuel injection system (10) comprises a body structure (11) defining a holding chamber (13) from which a fluid mixture contained therein can be subsequently delivered by a delivery injector (15) into the combustion chamber. The delivery injector (15) has an inlet (16) communicating with the holding chamber (13) and an outlet (18) for communication with the combustion chamber. The holding chamber (13) is adapted to receive a quantity of gaseous fuel from a fuel source through an inlet port (17). The holding chamber (13) is also adapted to receive a metered quantity of a supplementary gaseous fluid through a metering means (21). In one arrangement, the supplementary gaseous fluid may function as a diluent for the gaseous fuel.