Plumes of photochemical pollutants transported from the industrialized regions of the northeast United States and Canada were sampled over the North Atlantic Ocean at distances up to 1000 km from the coast. The plumes were found in well defined layers up to 1 km thick and were usually isolated from the surface by a low altitude inversion. Plume composition was consistent with the occurrence of extensive photochemical processing during transit from source regions as indicated by high O3 concentrations (O3 maximum ~150 parts per billion by volume (ppbv)), generally high fractional conversion (>85%) of NOx to its oxidation products, and high peroxide concentrations (median 3.6 ppbv; maximum 11 ppbv). These observations are in accord with processing times estimated from back trajectory analysis. CO and O3 concentrations were well correlated (r2=0.64) with a slope (0.26) similar to previous measurements in photochemically aged air. Good correlations were also observed between CO and accumulation mode particle number densities (r2=0.64), and CO and NOy (r2=0.67). O3 was found to depend nonlinearly on the NOx oxidation product concentration. At low values of (NOy-NOx), the slope (14) was within the range of values measured previously in photochemically aged air masses, at higher concentrations the slope was much lower (4.6). The low slope at high concentrations is attributed to minimization of losses of NOx oxidation products in spatially well-defined plumes during transport. A strong linear correlation (r2=0.73) was found between O3, and the concentration of radical sink species as represented by the quantity ((NOy-NOx)+2H2O2). ¿ American Geophysical Union 1996