Aircraft-borne multifrequency passive microwave observations of eight marine oil spills revealed that, in all cases, over 90 percent of the oil was confined in a compact region comprising less than 10 percent of the area of the visible slick. These measurements show that microwave radiometry offers a means for measuring the distribution of oil in sea-surface slicks; for locating the thick regions; and for measuring their volumes on an all-weather, day or night, and real-time basis.