Abstract: A diffuser (10) for controlling flow of air in an air conditioning system, the diffuser comprising two temperature sensitive elements (20, 22) for respectively sensing room temperature and duct temperature variations. The elements (20, 22) are axially aligned. Links (68) are provided for displacing an air flow control baffle (78). A control structure is provided for displacing the links (68) in dependence on sensed room temperature thereby to displace the baffle (68) and vary air flow. The control structure has two components (34, 36) which act on the links (68). When the element (22) is detecting cooled air the component (34) acts on the links (68). The component (36) acts on the links (68) when the element (22) is detecting warmed air. Switch over between these conditions occurs automatically as supply of air is changed from hot to cold and visa versa.