Measurements of NOy and the principal constituent NOy species (NO, NO2, PAN, HNO3, PPN, NO3-) were taken during the fall of 1993 at a high-elevation site in the Colorado mountains west of Boulder. The meteorology provided two principal flow regimes: strong westerly flow, bringing drier and cleaner air over the Continental Divide from the west where emission sources are sparse, and weaker easterly upslope flow which brought moist and more polluted air from the Denver-Boulder urban corridor and perhaps from smaller more local sources such as the mountain town of Nederland. The upslope flow data generally indicate a balance between measured NOy and the separately measured NOy species with NOx, PAN, and HNO3 accounting for over 90%. Under these conditions, NOx was the dominant fraction of NOy, and occasionally fresh emissions were indicated as shown by NOx/NOy close to unity. For downslope flow there was generally a shortfall in the NOy balance where measured NOy was higher than the constituent species by about 20%, on average. Known potential interferences in the measurements do not appear to account for the NOy deficit. Missing NOy correlated directly with NOx/NOy, O3, and aerosols and inversely with temperature. These relationships suggest the presence of one or more NOy species that were not measured by the individual techniques.¿ 1997 American Geophysical Union