To raise or lower the operating temperature of a fluidised bed to a desired level, the operating temperature of a second fluidised bed is raised or lowered to a temperature above or below that of the desired level and bed material is transferred between the two beds to bring the operating temperature of the first bed to the desired level. The first bed may be an incinerator and to lower the operating temperature the bed material of the second bed is cooled by a fluid in a heat exchanger and material is transferred between the two beds. The transfer of bed material may be brought about by a pump or one bed may be caused to overflow into the other with a corresponding counter-flow of bed material through a duct connecting the two beds.