Authentic cloud water samples absorb ultraviolet radiation and thereby initiate formation of peroxides (primarily hydrogen peroxide (H2O). This aqueous phase photo-formation of H2O2 is a widespread phenomenon in the terrestrial troposphere; midday equinox-normalized H2O2 photoformation rates ranged up to 3.0 μM/h for 36 different cloud water samples collected from sites in New York, North Carolina, Ontario, Virginia, and Washington. By comparison, previously published field studies have shown that over the eastern United States and Canada, approximately 100% of winter cloud water samples, 50--100% of spring and fall samples, and 3--7% of summer samples had peroxide concentrations less than 5 μM. Previously published model estimates of aqueous phase H2O2 formation rates in cloud drops from gas-to-drop transfer of H2O2 and HO•2 range from 1--10 μM/h for all but the most pristine regions of the troposphere. Based on a comparison of these published field and modeling results with the measured aqueous phase H2O2 photoformation rates reported here, we conclude that aqueous phase photochemistry is a significant and in some cases probably dominant source of H2O2 to tropospheric cloud drops. Theory predicts and experiments confirm that the initial rate of aqueous phase H2O2 photoformation is linearly dependent on solar actinic flux. The chromophores responsible for aqueous phase peroxide photoformation have not been identified, but there is evidence that organic chromophores are responsible for the H2O2 photoformation reported here. ¿ American Geophysical Union 1994