A gas cleaning separator for separating a flowable mixture of a gas and liquid; the separator comprising: a housing defining an inner space, a rotor assembly (78',84') for imparting a rotary motion onto said mixture of substances, the rotor assembly (78',84') being located in said inner space and rotatable about an axis relative to the housing, wherein the rotor assembly comprises a first inlet for receiving said mixture of substances, a first outlet from which said substances are ejected from the rotor assembly during use, and a first flow path for providing fluid communication between the first inlet and first outlet, wherein the first outlet is positioned more radially outward from said axis than the first inlet; and a housing member located adjacent the rotor assembly (78',84'), the housing member and the rotor assembly being spaced from one another so as to provide a first region therebetween on a first side of the housing member, said first region defining a first fluid flow route for cleaned gas ejected from the rotor assembly (78',84'); the housing member also being spaced from the housing so as to provide a second region therebetween on a second side of the housing member, said second region defining a second fluid flow route for oil and cleaned gas ejected from the rotor assembly (78',84'); characterised in that the rotor assembly (78',84') comprises first and second concentrically arranged members (86', 240) which each comprises a disk shaped portion, said first and second members (86',240) being centred on said axis, the disk shaped portions of said members (86', 240) each having a radially outer edge of a substantially circular shape, a second inlet which opens into said second region on said second side of the housing member, a second outlet positioned more radially outward from said axis than the second inlet, said second outlet being positioned between said first and second members (86', 240) at the radially outer edges thereof, and a second flow path for providing fluid communication between the second inlet and the second outlet, said second flow path comprising a space between said first and second members (86', 240), wherein at least one elongate element is located in said space between the first and second members (86',240) so as to move fluid located in said space outwards relative to said axis when, in use, the rotor assembly is rotated about said axis, and wherein the or each elongate element extends radially along the second flow path. a housing defining an inner space, a rotor assembly (78',84') for imparting a rotary motion onto said mixture of substances, the rotor assembly (78',84') being located in said inner space and rotatable about an axis relative to the housing, wherein the rotor assembly comprises a first inlet for receiving said mixture of substances, a first outlet from which said substances are ejected from the rotor assembly during use, and a first flow path for providing fluid communication between the first inlet and first outlet, wherein the first outlet is positioned more radially outward from said axis than the first inlet; and a housing member located adjacent the rotor assembly (78',84'), the housing member and the rotor assembly being spaced from one another so as to provide a first region therebetween on a first side of the housing member, said first region defining a first fluid flow route for cleaned gas ejected from the rotor assembly (78',84'); the housing member also being spaced from the housing so as to provide a second region therebetween on a second side of the housing member, said second region defining a second fluid flow route for oil and cleaned gas ejected from the rotor assembly (78',84'); characterised in that the rotor assembly (78',84') comprises first and second concentrically arranged members (86', 240) which each comprises a disk shaped portion, said first and second members (86',240) being centred on said axis, the disk shaped portions of said members (86', 240) each having a radially outer edge of a substantially circular shape, a second inlet which opens into said second region on said second side of the housing member, a second outlet positioned more radially outward from said axis than the second inlet, said second outlet being positioned between said first and second members (86', 240) at the radially outer edges thereof, and a second flow path for providing fluid communication between the second inlet and the second outlet, said second flow path comprising a space between said first and second members (86', 240), wherein at least one elongate element is located in said space between the first and second members (86',240) so as to move fluid located in said space outwards relative to said axis when, in use, the rotor assembly is rotated about said axis, and wherein the or each elongate element extends radially along the second flow path.