The results of an open sea experiment devised to investigate the feasibility of detecting and monitoring oil slicks by sonar through their effect on the acoustic backscatter signal are presented. The analysis, carried out over an angular range 10¿ to 70¿ from the horizontal, reveals a reduction in backscatter from the slick. The reduction in the mean backscatter ⟨&sgr;⟩ shows an angular dependence which reaches a maximum in the range 45¿ to 65¿. A gradual decrease of ⟨&sgr;⟩ as wind speed decreased and as the slick developed was also observed. Comparison of the acoustic data with Ku band radar reflection from an oil covered surface under similar wind speed conditions shows that differential radar backscatter from oil slicks may exceed that of sonar by a factor 5. ¿ American Geophysical Union 1992