Results of the analyses of SIR-C/X-SAR images from the test site North Sea as well as from other sea areas showing radar signatures of natural surface films and of oil spills are presented. Damping ratios (i.e., the ratios of the normalized radar cross section from a slick-free and a slick-covered water surface) and polarization signatures have been calculated and compared. A strong wind dependence of the damping ratio for the same substance is shown and explained by means of the source terms for the spectral energy in the ocean wave spectrum. It is therefore concluded that surface films of different visco-elastic properties can only be discriminated at low to moderate wind speeds. Furthermore, polarization signatures from film-covered as well as from film-free water surfaces have been calculated and compared. Differences between them are found only at low wind speeds and small incidence angles which is in accordance to Bragg scattering theory.