Dependence of microwave emission on oil type, age, film thickness, observational wavelength, antenna viewing angle and polarization was examined in the laboratory. Airborne measurements of controlled oil spills were subsequently performed at wavelengths of 8.1 and 3.2 mm for several refined and crude oil slicks, over a broad range of ocean surface and weather conditions. Oil slicks on the ocean surface provide unique and readily measurable signatures. Lowest amplitude passive microwave signals were of the order of 5 deg K. A 5 deg K microwave temperature difference is well within the detection capabilities of airborne passive microwave imaging systems.