Abstract: The invention concerns a method of operating a combustion plant comprising at least a first gas turbine (26), a second turbine (34, 50) and a membrane reactor device (16) which comprises a membrane filter device (18) for separating oxygen from a gas mixture and a combustion space (19). A gas mixture containing oxygen is supplied to the membrane reactor device (16). At least some oxygen is separated from the gas mixture with the help of the membrane filter device (18). The separated oxygen is used at the combustion in the combustion space (19). The oxygen-depleted gas that is obtained after the separation of oxygen and which has not gone through a combustion process is used to run the first gas turbine (26). Combustion gases run the second turbine (34, 50). The invention also concerns a combustion plant for carrying out the method.