In this study dual-polarized synthetic aperture radar (SAR) measurements were used to enhance oil spill observation. The co-polarized phase difference (CPD) was modelled and used to characterize the scattering return from oil spills and biogenic slicks. The model predicts, under low to moderate wind conditions, a larger CPD standard deviation (s) for oil with respect to the sea, while for biogenic slicks a s value similar to that for the sea is obtained. Experiments accomplished with multilook complex (MLC) C- and L-band SAR data show that the model predictions are confirmed and that the C-band is, as expected, to be preferred to the L-band.