The International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI) took place at Boulder, Colorado, from 15 to 19 June 1998 and offered the opportunity to test how well experimental techniques and theoretical models can determine the photolysis frequency of O3 → O(1D) in the troposphere. Different techniques measured the downwelling 2π sr component of j(O1D) at the ground and were blindly compared to each other. Moreover, theoretical j(O1D) model results were blindly compared to data measured under clear sky at relatively low aerosol optical density. Six experimental groups operated one chemical actinometer (CA), six spectroradiometers (SR), and four filter radiometers (FR). General good agreement with deviations less than 10% among the radiometers (SR and FR) was found for solar zenith angles (SZA) less than 60¿, provided that the instruments used similar absorption cross sections, quantum yields, and temperatures for deriving j(O1D). The deviations were generally larger at high solar zenith angles and reached a factor of 2 in some cases. Two spectroradiometers and one filter radiometer showed excellent agreement with each other and with the chemical actinometer at all solar zenith angles up to at least 80¿ within typically 5%. These radiometers used recently published O(1D) quantum yield data and explicitly considered the temperature dependence of j(O1D). This good agreement shows that each of the different categories of instruments (CA, SR, and FR) is in principle capable of accurate determinations of j(O1D). A large sensitivity was found to the choice of data for the O(1D) quantum yield. The best agreement between spectroradiometry and chemical actinometry was obtained when recently published quantum yield data were used. The IPMMI study thus supports the quantum yield recommendation by National Aeronautics and Space Administration-Jet Propulsion Laboratory <Sander et al., 2003> and International Union of Pure and Applied Chemistry (IUPAC) (http://www.iupac-kinetic.ch.cam.ac.uk; data sheet POx2 from 2001). Fifteen models that were operated by 12 model groups participated in the comparison of modeled j(O1D) with measured data. Most models agreed within 15% with the spectroradiometer-derived j(O1D) values under clear sky at SZA < 75¿, provided that they used similar absorption cross sections, quantum yields, and temperatures. While most models simulated the measured actinic flux quite well, significant deviations in j(O1D) were observed in cases when outdated O(1D) quantum yield data or inappropriate temperature data were used.