The extent to which H2O2 dissolved in cloud droplets is trapped in rime ice affects the composition of precipitation and the rate of H2O2 removal from the atmosphere. Measurements were conducted in winter stratiform clouds at a remote mountain-top site in southeastern Wyoming, thus avoiding the difficulties of preparing laboratory clouds whose chemical and physical properties are similar to natural clouds. Quantities directly observed were H2O2 concentrations in cloud water collected as rime, air temperature, gaseous H2O2, O3, and SO2 and cloud liquid water concentration. Values of the retention coefficient, &Ggr;, defined as the ratio of the H2O2 concentration in the melted rime sample divided by the equilibrium concentration in the supercooled droplets, were always less than unity (&Ggr;¿=0.24¿0.07). Corrections to account for the rapid reaction between dissolved H2O2 and sulfur (IV) increase the average value of the retention coefficient to only 0.30. An observed correlation between &Ggr; and riming rate suggests that H2O2 is released to the gas phase during riming. These field measurements do not agree with laboratory determinations of &Ggr;. ¿ American Geophysical Union 1992