Abstract: For determining or indicating the oil content of small amounts of oil in water a defined amount of water is removed so that the oil is concentrated in the water, and thereafter the oil content in the concentrated oil-water mixture is measured. So as to concentrate the oil, the oily water can pass a filter material, the color change or dielectric constant of which is measured. As an alternative, the oily water is concentrated in a rotating cyclone or centrifuge so that substantially all oil is accumulated in the center together with a smaller portion of the original water volume. The clarity of the water is then determined and considered as a measure of the oil content.