Atmospheric peroxides and formaldehyde were measured during the Polar Sunrise Experiment 1992 in Alert, Canada (82.5¿N, 62.3¿W). Two measurement periods, in the dark winter and in the sunlit spring, were chosen to investigate the effects of photochemistry. Continuous measurements were performed using diffusion scrubbers coupled to aqueous phase fluorometry. The concentration of total peroxides varied from below the detection limit (~10 parts per trillion by volume (pptv)) to 40 pptv in the dark and 100 to 400 pptv in the sunlit period with large variations in the ratio between H2O2 and organic peroxides. The CH2O concentrations measured in the dark were between 100 and 700 pptv and showed good correlation with a number of atmospheric constituents such as CH4, CO2, and Rn but anticorrelation to O3. A fraction of the observed CH2O concentrations is believed to be formed by nonphotochemical O3-alkene chemistry. In the presence of sunlight the CH2O concentrations ranged between 30 to 600 pptv without correlation to CH4, CO2, or Rn. The maximum CH2O concentrations were associated with air, depleted in O3, coming from the Arctic Ocean. During the O3 depletions, decreased peroxide concentrations were observed. The origin of the air mass was a very important factor during both periods in explaining the observed variabilities in CH2O and peroxide concentrations. ¿ American Geophysical Union 1994