A method for enhancing the operation of a circulating mass reactor, in which circulating mass reactor, at least a part of the heat contained by the flue gases formed in the circulating mass reactor is transferred to a fluidized material arranged to circulate in the circulating mass reactor, and which circulating mass reactor comprises - a fluidized-bed chamber, in the lower part of which is provided a fluidized bed containing the fluidized material, - means for separating the fluidized material from the flue gases, and - a return conduit system (15, 16, 19), through which the fluidized material can be returned to the fluidized-bed chamber and which includes at least one cooled return conduit (15, 16), in which a part of the heat energy contained by the fluidized material passing there through is transferred to a heat transfer liquid circulating in the circulating mass reactor by means of heat exchangers (115, 116) fitted in the cooled return conduits (15, 16), - a lower combustion chamber, which comprises the fluidized-bed chamber, and an upper combustion chamber, and a flow conduit connecting them, the flow conduit, the means for separating the fluidized material from the flue gases and the return conduit system (15, 16, 19) being arranged to be located between the lower combustion chamber and the upper combustion chamber, at least mainly above the lower combustion chamber and below the upper combustion chamber, the lower combustion chamber and the upper combustion chamber being dimensioned in such a way that the combustion of the fuel can be essentially completed before the discharge of the flue gases from the combustion chamber, wherein the average delay time of the flue gases in the upper combustion chamber is most preferably 0.3-3.0 seconds, and wherein the fluidized material is separated from the flue gases after the upper combustion chamber and guided back to the fluidized-bed chamber through the cooled return conduits (15, 16) and/or an uncooled return conduit system in a desired ratio. - a fluidized-bed chamber, in the lower part of which is provided a fluidized bed containing the fluidized material, - means for separating the fluidized material from the flue gases, and - a return conduit system (15, 16, 19), through which the fluidized material can be returned to the fluidized-bed chamber and which includes at least one cooled return conduit (15, 16), in which a part of the heat energy contained by the fluidized material passing there through is transferred to a heat transfer liquid circulating in the circulating mass reactor by means of heat exchangers (115, 116) fitted in the cooled return conduits (15, 16), - a lower combustion chamber, which comprises the fluidized-bed chamber, and an upper combustion chamber, and a flow conduit connecting them, the flow conduit, the means for separating the fluidized material from the flue gases and the return conduit system (15, 16, 19) being arranged to be located between the lower combustion chamber and the upper combustion chamber, at least mainly above the lower combustion chamber and below the upper combustion chamber, the lower combustion chamber and the upper combustion chamber being dimensioned in such a way that the combustion of the fuel can be essentially completed before the discharge of the flue gases from the combustion chamber,