Abstract: A thermally controlled diffuser comprises a means for connecting the diffuser to a duct, a divergent outlet, a means for deflecting flow from the divergent outlet, and a means for supporting the flow deflection means in a predetermined positional relationship with respect to the divergent outlet. The support means includes a temperature sensitive mechanism whereby the predetermined positional relationship between the outlet and the flow detection means may be varied depending on a temperature sensed by the temperature sensitive mechanism so as to produce a variation in the outlet flow pattern of the diffuser. The thermally controlled diffuser may preferably be a ceiling diffuser, and preferably the variation in the positional relationship is in a vertical direction when the diffuser is installed. A method of automatically varying the flow pattern of a diffuser is also described.