Experiments are reported that were designed to investigate the differences in diffusion between a plume from an obstacle in the ocean and a simultaneous plume from an undisturbed oceanic site. In the absence of an actual ocean-based structure from which to conduct experiments, a small island was chosen as the best available substitute. Great Gull Island is located northeast of Long Island, 3.5 km from the nearest land, a larger island. Overwater fetch in other directions is 8 to 20 km. The island is 800 x 175m, tapering at both ends. It is hilly and rocky and contains numerous remnants of the earlier concrete fortifications and rock walls. Maximum elevation is about 12 m and the tallest structure about 18 m. Simultaneous releases of oil-fog smoke were made from the island and from a small boat anchored in a nearby position undisturbed by flow around the island. The center, upwind side and downwind side of the island were each used as source locations. The widths of the plumes and their concentration distributions at one or more downwind distances were measured by an instrument in a second boat. Extensive series of photographs were taken from the island, the second boat and an aircraft. Concurrent meteorological measurements were taken from two locations on the island, from the boats and from the aircraft. A series of experiments was made in September 1974 when low level lapse rates over the ocean were mostly unstable and another series in April and May, 1975, under neutral and stable conditions.