Measurements of ozone, hydrogen peroxide, methylhydroperoxide, and formaldehyde in the marine boundary layer of the tropical southern Atlantic Ocean were performed on board the research vessel Polarstern during its cruise from Bremerhaven, Germany (54 ¿N, 8 ¿E), to Punta Arenas, Chile (53 ¿S, 71 ¿W), in October and November 1994. Model calculations were made to estimate the deposition rates of H2O2 and CH3OOH. The modeling required that long-range transport and advection processes of photochemical aged air masses that originated in African biomass burning plumes be taken into account. For H2O2 the dry deposition rate was estimated to be 1¿10-5/s and the air-sea flux was estimated to be 4.5¿1010 molecules/cm2 s, while for CH3OOH the dry deposition and the air-sea flux were estimated to be 0.8¿10-5/s and 3¿1010 molecules/cm2 s, respectively. These values compared well with other estimates. Because of the uncertainties in the rate constants for the photochemical production and loss of methyl hydrogen peroxide the accuracy of the calculated deposition rates for both CH3OOH and H2O2 is limited. These chemical rate constants are not accurate enough to provide an unambiguous estimate of the deposition rate constants through simulation of field observations. ¿ 1999 American Geophysical Union