The relationship between photolysis frequencies derived from spectroscopic measurements of actinic fluxes and irradiances was determined during a coordinated measurement campaign (International Photolysis Frequency Measurement and Modeling Intercomparison campaign (IPMMI)). When differences in viewing geometries are taken into account, the measurements are in close agreement. An empirical relationship, which is useful for high sun (noon) conditions or for daily integrals, was found to convert irradiance data to photolysis frequencies. For low-sun conditions (large solar zenith angle), model calculations were shown to improve the accuracy. However, the input parameters to the model are site specific and the conversion depends on diffuse/direct ratios. During cloudy conditions, significant improvements in the conversion can be achieved by assuming the radiation field to comprise entirely diffuse isotropic radiation when the UVA transmission by cloud is less than 0.8. Changing cloud conditions remain the greatest limitation, but they tend to bias the results away from the clear-sky case in a systematic way. Furthermore, although the cloud effects on the photolysis rates of nitrogen dioxide (J(NO2)) are rather large, they are much smaller for ozone photolysis (J(O3 → O(1D))), which is of prime importance in tropospheric chemistry. The study shows the potential for deriving historical and geographical differences in actinic fluxes from the extensive records of ground-based measurements of spectral irradiance.