Abstract.The air chemistry investigations, performed on R/V Polarstern during the expedition ARK X/1 in July/August 1994, comprised a mapping of the vertical ozone distribution by ozone soundings and the surface mixing ratios of H2O2 and NO above the East Greenland Sea between 70°N and 80°N. The observed H2O2 mixing ratios varied from about 100 parts per trillion by volume (pptv) up to around 1000 pptv. Surface ozone concentrations can be correlated with the source region of the advected air masses. Ozone mixing ratios from around 33 parts per billion by volume (ppbv) up to 60 ppbv have been observed in air parcels emanating from industrialized regions. In contrast, pristine Arctic air masses showed low ozone mixing ratios of about 23 ppbv. The tropospheric part of the vertical ozone profiles occasionally exhibited significant maxima above the planetary boundary layer. Apart from cases where intrusions of stratospheric ozone were obvious, photochemical ozone formation along trajectories originating from polluted regions was most probably the reason for layers with enhanced ozone concentrations. The measured surface NO mixing ratios were extremely low, typically between 3 and 8 pptv. Thus local photochemical surface ozone production could be largely ruled out. In conclusion it can be stated that in summer, intrusion of ozone rich stratospheric air and advection of polluted air masses from lower latitudes appeared to be the dominant ozone sources of the marine Arctic troposphere.