The in situ photolysis frequencies for 11 molecules were determined using new actinic flux spectroradiometer systems mounted on the NASA DC-8 research aircraft during the Pacific Exploratory Mission (PEM)-Tropics mission. Photolysis frequencies for O3, NO2, HONO, CH2O, H2O2, CH3OOH, HNO3, PAN, CH3NO3, CH3CH2NO3, and acetone were calculated from the 30 s averaged actinic flux measurements. The accuracy of the actinic flux measurements was approximately ¿11.5% in the UV-B range and 8% in the UV-A range. Uncertainties of the reported photolysis frequencies vary between ¿15% and ¿20% dependent on the quality of the molecular absorption cross section and quantum yield data. Approximately 139 hours of data were taken during 17 flights over the Pacific Ocean, and photolysis frequencies have been reported to the mission archive. During the mission, latitudes range from 45 ¿N to 72.5 ¿S, the longitude ranges from 10 ¿W to 173 ¿E, and the altitude ranges from sea level to 11.9 km. The geographical extents covered, combined with local times from sunrise to sunset, encompass solar zenith angles between 1¿ and >90¿ resulting in a broad range of photolysis frequencies. Persistent scattered clouds created photolysis frequency enhancements of approximately a factor of 2 over clear-sky values and reductions of greater than 90% of clear-sky values for portions of the mission. ¿ 1999 American Geophysical Union