AbstractGas phase H2O2, organic peroxides, and formaldehyde (HCHO) have been measured in situ during October/November 1996 on board RV Polarstern in surface air over the Atlantic from 48°N-35°S with different analytical methods. The results indicate that recombination and self-reactions of peroxy radicals largely dominate over scavenging by NO. The peroxy radical chemistry was governed by the photooxidation of CH4 and CO, as could be deduced from our failure to detect organic hydroperoxides other than CH3OOH (methyl hydroperoxide (MHP)). Hydroperoxide and formaldehyde mixing ratios were highest within the tropics with peak values of around 2000 parts per trillion by volume (pptv) (H2O2), 1500 pptv (MHP), and 1000 pptv (HCHO). In the case of H2O2 and MHP we observed diurnal variations of the mixing ratios in the tropical North Atlantic and derived deposition rates of around (1.8±0.6) 10-5 s-1 for H2O2 and (1.2±0.4) 10-5 s-1 for MHP. The measured MHP/(H2O2+MHP) and MHP/HCHO ratios corresponded to 0.32±0.12 and 0.87±0.4, respectively. HCHO mixing ratios observed during the expedition were significantly higher than predicted by current photochemical theory based on the photooxidation of CH4 and CO.