A method of and apparatus for controlling heat transfer in a fluidized bed reactor having a heat transfer chamber (312) with a bed (314) of solid particles therein, means (320,322) for introducing fluidizing gas into the heat transfer chamber for fluidizing the bed of solid particles therein and heat transfer surfaces (316) in contact with the bed of solid particles in the heat transfer chamber. Heat is transferred to said heat transfer surfaces from the solid particles. The fluidization of the bed of solid particles is varied according to a periodical function, e.g. by control means (34) periodically varying the flow velocity of fluidizing gas being introduced into the heat transfer chamber. Thereby the instantaneous heat transfer, as well as, the effective heat transfer from solid particles to the heat transfer surfaces may be controlled.