Measurements of tropospheric H2O2 and O3 concentrations were made over the Atlantic Ocean near the U.S. east coast (southeast of Newport News, Virginia) and near Bermuda during July 1988 with an instrumented aircraft. Oxidant concentrations were high (O3 up to 100 parts per billion by volume (ppbv) and H2O2 up to 2.5 ppbv) near the U.S. east coast at low altitude but decreased rapidly with increasing offshore distance to concentrations comparable to those measured near Bermuda. O3 concentrations observed near Bermuda at low altitude were 14–22 ppbv, comparable to those reported for remote subtropical marine locations. O3 concentrations increased with altitude (up to 2.6 km); high-altitude concentrations were 30–80 ppbv. H2O2 concentrations near Bermuda varied by a factor of 2, 0.7–1.5 ppbv, from day to day, but were nearly constant at all altitudes during individual flights. Because a high-pressure system centered over the western Atlantic Ocean limited eastward advection of continental air toward Bermuda, these concentrations are assumed to be typical of mid-latitude marine air during summer.