Ozonolysis of C2H4 was carried out in a 580 l glass reaction vessel at 1--5 ppm reactant concentrations, with added water vapor. Under dry conditions ([H2O>0=0.5 ppm), HCHO, CO, CO2, (CHO)2O (formic acid anhydride), H2O2, and CH3OOH were identified as the reaction products. Under wet conditions ([H2O>0=2¿104 ppm), HCOOH yields approaching ca. 20% of the converted C2H4, were observed, while no (CHO)2O was formed. Hydroxymethyl hydroperoxide, HOCH2OOH, was observed as the major peroxide, and found to be formed only in the presence of water vapor. Direct reactions of H2O vapor with the excited CH2OO* radicals and with stabilized CH2OO radicals are postulated to explain the formation of HCOOH and HOCH2OOH in the presence of water vapor, respectively. ¿ American Geophysical Union 1994