The photostationary state (PSS) involving NO, NO2, and O3 has been studied at a site located in the Colorado mountains. During the daytime in summer the concentration of NO2 relative to the concentration of NO is greater than expected from the PSS. These results indicate that an additional oxidant OX other than O3 is converting NO to NO2. The level of OX depends on season, NOx(NO+NO2) concentration, and solar UV intensity. The maximum OX levels occur for an NOx mixing ratio of about 0.4 parts per billion by volume (ppbv). For NOx mixing ratios below 0.1 ppbv, no substantial imbalance in the PSS is observed. For NOx concentrations between 0.25 and 1.0 ppbv the OX level depends linearly on solar UV intensity. In the winter the level of OX is much lower than in the summer for all NOx concentrations and solar UV intensities. The nature of OX, as indicated from the NOx-catalyzed formation of O3, is examined.