Measurements of NO and NOy using a chemiluminescence technique were made on board a DC-8 aircraft during NASA's Pacific Exploratory Mission-West B (PEM-West B). The gold converter to convert NOy species into NO was operated at a constant pressure using a servo-controlled Teflon valve, which has been used for NOy measurements on board the ER-2 aircraft. The results of laboratory tests and some flight data during PEM-West B are presented. These experiments indicate no detectable inlet loss of HNO3 in dry air, although some loss was observed at H2O mixing ratios of 1--2%. The laboratory tests also showed small variability in the NOy artifact, high conversion efficiency for NO2 and HNO3, low HCN conversion efficiency, good repeatability of the measurements, and fast response. The control of the converter pressure during flight has been proven to be very advantageous in making reliable aircraft NOy measurements in the troposphere. The uncertainties of the NO and NOy PEM-West B data, including the effects of HCN conversion and HNO3 inlet loss, have been estimated. ¿ 1997 American Geophysical Union