An air diffuser for supplying air to a space, the diffuser having a central axis (I, A) and comprising; a plurality of discharge elements arranged to guide an air stream towards the space, the plurality of discharge elements having respective edge regions that define a face of the diffuser; wherein a plurality of channels (25, 115) is located about the diffuser central axis, each channel being formed between adjacent pairs of discharge elements and configured to guide the air to the space; an adjustment mechanism (8, 111, 113, 511, 513, 611, 811, 813, 911, 913) able to translate along, and rotate about, the central axis between an advanced position and a retracted position; wherein the adjustment mechanism comprises a plurality of substantially radially aligned guide vanes (12, 123, 523, 623, 823), each guide vane having a respective discharge element (7, 117) and being configured so that, in use, each guide vane and adjacent peripheral portion of its respective discharge element can together form a diffuser blade, characterized in that the diffuser is further configured so that when the adjustment mechanism is in the retracted position, each guide vane is positioned such that it forms an extension of a peripheral portion of its respective discharge element to thereby increase a guidance width of the diffuser blade, and when the adjustment mechanism is in the advanced position, each guide vane is positioned over the peripheral portion of its respective diffuser element to thereby decrease the guidance width of the diffuser blade, and when the adjustment mechanism is in the retracted position the adjustment mechanism is positioned away from the diffuser face and adjacent to the channels such that the channels are unobstructed by the adjustment mechanism, and when the adjustment mechanism is in the advanced position, the adjustment mechanism is positioned towards the diffuser face such that it obstructs the channels. a plurality of discharge elements arranged to guide an air stream towards the space, the plurality of discharge elements having respective edge regions that define a face of the diffuser; wherein a plurality of channels (25, 115) is located about the diffuser central axis, each channel being formed between adjacent pairs of discharge elements and configured to guide the air to the space; an adjustment mechanism (8, 111, 113, 511, 513, 611, 811, 813, 911, 913) able to translate along, and rotate about, the central axis between an advanced position and a retracted position; wherein the adjustment mechanism comprises a plurality of substantially radially aligned guide vanes (12, 123, 523, 623, 823), each guide vane having a respective discharge element (7, 117) and being configured so that, in use, each guide vane and adjacent peripheral portion of its respective discharge element can together form a diffuser blade, characterized in that the diffuser is further configured so that when the adjustment mechanism is in the retracted position, each guide vane is positioned such that it forms an extension of a peripheral portion of its respective discharge element to thereby increase a guidance width of the diffuser blade, and when the adjustment mechanism is in the advanced position, each guide vane is positioned over the peripheral portion of its respective diffuser element to thereby decrease the guidance width of the diffuser blade, and when the adjustment mechanism is in the retracted position the adjustment mechanism is positioned away from the diffuser face and adjacent to the channels such that the channels are unobstructed by the adjustment mechanism, and when the adjustment mechanism is in the advanced position, the adjustment mechanism is positioned towards the diffuser face such that it obstructs the channels.