Abstract: A diffuser has an upstream duct, a downstream duct, the adjacent ends of the ducts defining a sudden enlargement of flow area. A fence arranged downstream of the downstream end of the upstream duct defines the upstream end of the downstream duct and has a free edge defining a flow area intermediate between that of defined by the adjacent ends of the two ducts. A chamber at the outside of the upstream duct has an opening defined by the downstream end of the upstream duct and the free edge of the fence. The ratio between the flow areas of the two ducts of their adjacent ends lies between 1.4 and a minimum greater than 1. The above ratio provides high diffuser effectiveness with little or no bleed from the chamber. Elements each comprising a diffuser as above may be arranged in flow-series to make possible a pressure rise greater than that given by a single element.