A method and apparatus for treating hot process gases produced in high temperature processes in a circulating fluidized bed reactor. The reactor (10) includes a mixing chamber (12), a particle separator (16) and a return duct (18) for returning the circulating mass from the particle separator to the mixing chamber. The return opening (24) of the return duct is so arranged that the flow of solids entering the mixing chamber via the opening is directed substantially downwardly. The inlet (22) for hot process gas is so arranged as to allow the hot process gas to flow into the mixing chamber as a substantially upwardly directed solids flow so that the solids flow comes into contact with the gas flow.