Hydrogen peroxide (H2O2) contributes to the atmosphere's oxidizing capacity, which determines the lifetime of atmospheric trace species. Measured bidirectional summertime H2O2 fluxes from the snowpack at Summit, Greenland, in June 1996 reveal a daytime H2O2 release from the surface snow reservoir and a partial redeposition at night. The observations also provide the first direct evidence of a strong net summertime H2O2 release from the snowpack, enhancing average boundary layer H2O2 concentrations approximately sevenfold and the OH and HO2 concentrations by 70% and 50%, respectively, relative to that estimated from photochemical modeling in the absence of the snowpack source. The total H2O2 release over a 12-day period was of the order of 5¿1013 molecules m-2s-1 and compares well with observed concentration changes in the top snow layer. Photochemical and air-snow interaction modeling indicate that the net snowpack release is driven by temperature-induced uptake and release of H2O2 as deposited snow, which is supersaturated with respect to ice-air partitioning, approaches equilibrium. The results show that the physical cycling of H2O2 and possibly other volatile species is a key to understanding snowpacks as complex physical-photochemical reactors and has far reaching implications for the interpretation of ice core records as well as for the photochemistry in polar regions and in the vicinity of snowpacks in general. ¿ 2001 American Geophysical Union