At a site near Boulder, Colorado, simultaneous atmospheric measurements were made of NO, NOx, and NOy in a field intercomparison of instruments involving two currently employed techniques of NOx and NOy measurement. Both NOy instruments depended upon the reduction of the NOy species to NO with detection by chemiluminescence, but different catalysts were employed for the reduction: (1) a gold catalyst (with addition of 0.3% CO) at 300 ¿C, or (2) molybdenum catalyst at 400 ¿C. The two systems of NOx detection involved (1) photolysis of NO2, and (2) reduction of NO2 by solid ferrous sulfate. Several times during the intercomparison the response to calibrated samples of reference compounds NO, NO2, PAN, HNO3, n-propyl nitrate (NPN), and NH3 was measured. From the results the following conclusions were made: (1) The two NO instruments agreed on NO mixing ratios that were measured during the daytime hours over a range from the limits of detection to 35 parts per billion by volume (ppbv), (2) the two NOy instruments gave similar estimates of NOy in ambient air over a wide range of mixing ratios (0.4--10 ppbv), and (3) the ferrous sulfate converter used for NOx detection showed a significant interference from NPN and PAN. ¿ American Geophysical Union 1987