Abstract: A separator for separating gas from oil in a mixture thereof such as at an oil well head comprises one or a plurality of separator devices (13) each basically comprising a foraminous wall constituted by a sleeve (22;56) surrounding a closely fitting solid core (23) having a plurality of helical channels defining flattened flow paths bounded on a major face by the foraminous sleeve (22;56) so that gas can escape to a gas gallery (20) while the oil tends to pass along the helical channels to an oil collector (16). A secondary outer foraminous sleeve (57) may be provided to vary the effective resistance offered by the foraminous wall by relative displacement with respect to the first mentioned sleeve (56) causing variation in the overlap of the apertures in the two sleeves.