Hot solids coming from a fluidized bed reactor are passed through the cooling apparatus and at least in part are recycled to the fluidized bed reactor. In the cooling apparatus the hot solids flow first through a first fluidized bed and then flow downwardly through a gas-collecting space to a second fluidized bed, which is provided with cooling coils and in which the solids flow downwardly to a solids transfer passage and are then raised through a third fluidized bed to a discharge line. The system is controlled to cause solids coming from the first fluidized bed to flow through the gas-collecting space directly to the discharge line.