The damping of ocean surface waves by a monomolecular oleyl alcohol film of about 1.5--3 km2 in area is measured in the North Sea by wave staffs, a coherent X band microwave scatterometer mounted on a sea-based platform, and an incoherent Ku band microwave scatterometer carried by an aircraft under moderate wind conditions (wind speed u10 = 3.5--7.7 m s-1). The observed wave attenuation by the monomolecular surface film measured by a wave staff in the frequency band between 3.2 and 16 Hz is in the range of about 40--60%, with only a slight increase with frequency. From this result it can be predicted that slicks affect microwave backscattering similarly in the L band (l0≈20 cm) as in the Ku and X bands (l0≈2 cm). It is shown by additional wave tank experiments that a direct influence of oleyl alcohol surface films on wave damping is confined to frequencies f?2 Hz, but a further indirect effect of oleyl alcohol films on the damping of ocean waves in the frequency range between 0.12 and 0.7 Hz by modifying the wind input and wave-wave interaction mechanisms is indicated from our results. A possible directional dependence of the wave-damping effect caused by surface films is discussed.