Experiments to simulate cloudwater conditions were carried out by using ambient aerosol samples suspended in an aqueous solution. Electron donors known to exist in atmospheric cloudwater (oxalate, formate, or acetate) were then added to the simulated cloudwater, and the solution irradiated with ultraviolet light while important species were measured (i.e., H 2O 2, Fe total, Fe(II) aq, and pH). A total of four different ambient aerosol samples were used in the simulated cloudwater experiments; they were collected from (1) Whiteface Mountain, NY,(2) Pasadena, CA, and (1) Sequoia National Park, CA. Hydrogen peroxide (H 2O 2) photoproduction was observed in the simulated cloudwater experiments with added oxalate. Fe(II) aq photoproduction was observed in the simulated cloudwater experiments with and without added acetate or added formate using ambient aerosol collected simultaneously with the ambient aerosol used in the added oxalate experiments. The production of Fe(II) aq showed that Fe from the ambient aerosol was available for photochemical redox reactions. In all cases, the production rates for Fe(II) aq and H 2O 2 in the light were greater than production rates in nonirradiated control experiments. The simulated cloudwater experiments (with four different aerosol samples) showed similar behavior to previous experiments carried out with synthetic Fe-oxyhydroxy polymorphs in the presence of oxalate, formate, or acetate. The Fe present in the ambient aerosol appears to be a critical component for the production of H 2O 2 in the simulated cloudwater experiments.