An apparatus for separating a fluid mixture includes a uniaxial cyclonic separator (2) having a separation chamber (18) for separating the fluid mixture by cyclonic action into a first fluid and a second fluid. An inlet (16) is located at a first end of the separation chamber (18) for receiving a fluid mixture, while a first outlet (22) for the first fluid and a second outlet (26) for the second fluid are located at a second end of the separation chamber. A gas injection means (12) is provided for injecting a gas into the fluid mixture to aid separation within the separation chamber (18). The gas injection may also be through an annular chamber surrounding the separator chamber (18). The gas in this case is introduced through a porous medium 130 via a gas supply line 136.