We present an analysis of UV radiative transfer under cloudy conditions in relation to the photochemistry of the hydroxyl radical (OH) by means of modeling and measurements. The measurements, which are part of the First Aerosol Characterization Experiment (ACE 1) campaign, consist of four different ascents/descents selected from research flight (RF) 12 and 28. The ascents/descents give vertical profiles of UV irradiances, microphysical properties, standard meteorological parameters, and OH concentration in the presence of one or more cloud layers. In order to assess the photochemical conditions for these (complex) cloudy cases we first compare the UV irradiance measurements with modeled profiles. We model the UV irradiances using cloud optical properties which we derive from the measured microphysical properties. Second, we use the simultaneously modeled actinic flux to calculate the rate constant of the photodissociation of ozone to the O(1D) radical (JO3). This reaction initiates the primary OH-production. Finally, we compare the measured OH concentrations with those derived from the radiative transfer calculations. For single-cloud layer cases we successfully simulated UV radiative transfer, JO3 and OH. For more complex multiple-cloud layer cases the UV radiative transfer could only be explained allowing large variations of the cloud optical thickness (from zero to double the measurement derived values). The impact of such variations on the modeled radiation-derived photochemical properties, JO3 and OH, was, with a variation of 25%, found to be relatively small. As a consequence, we were able to simulate the general profile of OH for these complex cloudy conditions. ¿ 1998 American Geophysical Union