The effect of free gas on cyclonic oil-water separation was examined using a geometry which sought to minimize problems with gas. Tests were carried out using the purpose built oil-water separation facility at Bradford University where pre-choke conditions could be partially simulated. Firstly, tests were carried out with water and gas-water to look at flow behaviour and control parameters, then comparative separation tests carried out with gas-oil-water. Comparisons were also made with test data from a conventional de-oiling hydrocyclone. The effects of gas content, feed drop size distribution, overflow size diameter and feed pressure were all investigated. A threshold effect of free gas on flow stability and separation efficiency was observed. The see-through hydrocyclone allowed the collection of semiquantitative data on gas core diameter and behaviour and on gas slip velocity at inlet.