Reflected optical second-harmonic generation (SHG) was applied for the first time to in situ study of the air-sea interface. SHG signals were detected from the first several molecular layers of the ocean surface during the SAR X Band Ocean Nonlinearities (SAXON) Chesapeake Light Tower experiment in 1988, and during experiments on Nantucket Sound at the Woods Hole Oceanographic Institution in 1989. The SHG response of the ocean surface was observed to correlate with increased slick activity and surface tension measurements of surface water organic content. The SHG response was similar for naturally occurring slicks and for artificially created slicks of several known materials. The SHG signal intensity was also used to estimate the second-order nonlinear optical susceptibility of the ocean surface. It was determined that the SHG nonlinear laser technique is a useful noninvasive probe for in situ studies of ocean surface chemistry. ¿ American Geophysical Union 1992