Hydrogen peroxide concrntrations have been determined in marine rain from the Gulf of Mexico (114--820¿10-7 mol L-1, x¿=402¿10-7 mol L-1 s=218¿10-7 mol L-1), the western Atlantic Ocean (84--206¿10-7 mol L-1,x¿=127¿10-7 mol L-1, s=45¿10-7 mol L-1), and one rain event off the Florida Keys (x¿=284¿10-7 mol L-1, s=38¿10-7 mol L-1). In several cases, simultaneous measurements of the concentration of H2O2 in the surface ocean were also determined. These measurements were made with the ship under way using a continuous flow sampling system with the intake at the bow. In shallow stratified layers, rain events can increase the existing hydrogen peroxide concentration by a factor greater than 10 and dramatically increase the mean H2O2 concentration of mixed depth layers of 50 m or more. Rain is a significant transient source of H2O2 in the surface ocean. ¿American Geophysical Union 1987