Abstract. The performance of the EPA Federal Equivalent Method (FEM) technique for monitoring ambient concentrations of O₃ via ultraviolet absorption (UV) has been evaluated using data from the Mexico City Metropolitan Area field campaign (MCMA-2003). Comparisons of UV O₃ monitors with open path Differential Optical Absorption Spectroscopy (DOAS) and open path Fourier Transform Infrared (FTIR) spectroscopy instruments in two locations revealed average discrepancies in the measured concentrations between +13% to −18%. Good agreement of two separate open path DOAS measurements at one location indicated that spatial and temporal inhomogeneities were not substantially influencing comparisons of the point sampling and open path instruments. The poor agreement between the UV O₃ monitors and the open path instruments was attributed to incorrect calibration factors for the UV monitors, although interferences could not be completely ruled out. Applying a linear correction to these calibration factors results in excellent agreement of the UV O₃ monitors with the co-located open path measurements; regression slopes of 0.94 to 1.04 and associated R² values of >0.89. A third UV O₃ monitor suffered from large spurious interferences, which were attributed to extinction of UV radiation within the monitor by fine particles (<0.2 µm). The overall performance of this particular monitor was poor owing to a combination of interferences from a contaminated particle filter and/or ozone scrubber. Suggestions for improved operation practices of these UV O₃ monitors and recommendations for future testing are made.