A fuel system comprises a fuel actuation arrangement 28 operable to split a metered flow of fuel into at least a first stream and a second stream, a control unit 25 controlling the operation of the fuel actuation arrangement 28, and wherein the control unit 25 controls the operation of the fuel actuation arrangement 28 in response to the output of a temperature sensor 34 sensitive to a gas temperature downstream of the high pressure compressor 14 of an associated engine and the output of a gas sensor 24 sensitive to at least one parameter of the composition of a gas downstream of a combustor 15 of the engine.