Radar signatures of mineral oil spills consisting of heavy and light fuel were measured by an airborne five-frequency ( L - S - C - X - and K -band) multi u polarization microwave scatterometer flown on a helicopter during a controlled oil spill experiment in the North Sea. The damping ratio, defined as the ratio of the backscattered radar power from an oil-free and an oil-covered sea surface, was measured at different radar frequencies and incidence angles such that the Bragg wavenumbers, k, between 20 radm 1 and 500 radm 1 were covered. The B following results were obtained: for the five oil spills deployed in the experiment the damping ratio, in general, increases monotonically from k 20 radm 1 to B k 500 radm 1 . At S - C - X - and K -band, the damping ratio is larger for heavy B u fuel than for light fuel spills, while at L- band it is almost the same. For heavy fuel, the damping ratio increases with increasing thickness of the oil layer. Furthermore, for wind speeds between 6 m s 1 and 10 m s 1 the damping of the backscattered radar power is independent of the polarization of the radar and its look direction relative to the wind. The results obtained in the present investigation agree well with those obtained from similar measurements with heavy fuel under high wind conditions.