Selected trace gas mixing ratios (i.e., peroxy radicals (RO*2 = HO2 + ΣRO2), nonmethane hydrocarbons (NMHCs), O3, CO, HCHO, and NO) and photolysis rate coefficients of j(NO2) and j(O(1D)) were measured in the marine boundary layer (MBL) over the Indian Ocean. The measurements were performed during February, March, and April 1999 as a part of the Indian Ocean Experiment (INDOEX) on board the research vessel R/V Ronald H. Brown. During the campaign, air parcels having different origins and consequently variable compositions were encountered, but all air masses, including those heavily polluted with NMHCs and aerosols, were in the regime of rapid photochemical ozone destruction. The influence of aerosols on the photolysis frequencies was investigated by comparison of measurements and results from the radiative transfer model PHOTOST: the high optical depth (up to 0.6) and low single scattering albedo of the aerosol reduces the UV flux at the surface substantially downwind of India and Arabia causing, for instance, a reduction in j(O(1D)) by up to 40%. The diurnal behavior of the trace gases and parameters in the MBL has been investigated by using a time-dependent zero-dimensional chemical model. Significant differences between the diurnal behavior of RO*2 derived from the model and observed in measurements were identified. The measured HCHO concentrations differed from the model results and are best explained by some missing chemistry involving low amounts of Cl. Other possible processes describing these two effects are presented and discussed.