A method is provided for operating a gas turbine installation which has at least one compressor for compressing combustion air, at least one combustion chamber for combusting a supplied fuel, using the compressed combustion air, and also at least one turbine which is exposed to throughflow by the hot gases from the at least one combustion chamber. Both a first fuel on a carbon base, especially in the form of natural gas, and also a second fuel, in the form of a hydrogen-rich fuel or pure hydrogen, are used as fuel. A reduction of the CO2 emission without basic modifications to the installation is achieved by the first and the second fuels being intermixed and combusted together in the at least one combustion chamber.