A gas turbine engine system comprising: a first fuel store of a first fuel; a second fuel store of a second fuel; an engine arranged to produce mechanical power by combustion or oxidation of fuel in the engine; a fuel distribution system arranged to deliver fuel from the first and second fuel stores (106, 108) to the engine, the first fuel delivered at a first mass flow rate, the second fuel delivered at a second mass flow rate, the first and second mass flow rates contributing to a total mass flow rate of fuel to the engine; and a control system arranged to control the relative fractions of the total mass flow rate of fuel to the engine represented by the first and second mass flow rates according to the required power output of the engine such that the relative fraction of the total mass flow rate of fuel to the engine represented by the second mass flow rate increases with increasing required power output of the engine for at least part of a period of operation of the engine, wherein the first and second fuels are selected such that operation of the engine using only the second fuel results in a lower engine temperature than operation of the engine using only the first fuel, when producing the same amount of mechanical power and/or wherein the second fuel has a lower specific energy than the first fuel and/or wherein the second fuel produces more water during combustion than the first fuel per unit of fuel energy released during combustion; characterised in that the control system is arranged to, during a period of operation of the engine, control the relative fractions of the total mass flow rate of fuel represented by the first mass flow rate and the second mass flow rate, based on an engine temperature, and the control system is arranged to control the relative fractions of the total mass flow rate of fuel represented by the first mass flow rate and the second mass flow rate to change the engine temperature at a first rate of change, whilst maintaining constant mechanical power, and wherein the control system is arranged to vary the engine temperature at constant mechanical power prior to a period of increased or decreased mechanical power demand to pre-warm or pre-cool the engine, wherein the first rate of change is slower than the rate of change of engine temperature resulting from the change in mechanical power demand without pre-warming or pre-cooling. a first fuel store of a first fuel; a second fuel store of a second fuel; an engine arranged to produce mechanical power by combustion or oxidation of fuel in the engine; a fuel distribution system arranged to deliver fuel from the first and second fuel stores (106, 108) to the engine, the first fuel delivered at a first mass flow rate, the second fuel delivered at a second mass flow rate, the first and second mass flow rates contributing to a total mass flow rate of fuel to the engine; and a control system arranged to control the relative fractions of the total mass flow rate of fuel to the engine represented by the first and second mass flow rates according to the required power output of the engine such that the relative fraction of the total mass flow rate of fuel to the engine represented by the second mass flow rate increases with increasing required power output of the engine for at least part of a period of operation of the engine,