A chemiluminescence analyzer was used to measure NOx mixing ratios at the west coast of Ireland. Two measurement modes allowed the determination of NO and NOx = NO+NO2. In a third mode using a molybdenum converter, higher signals were observed than was in the second mode indicating that nitrogen compounds other than NO+NO2 are registered. They are denoted 'excess NOx'. The average NO2 mixing ratio for a week period was 101¿87 pptv. In pure marine air masses identified by means of trajectory calculations, the NO2 mixing ratios were lower and exhibited in addition a diurnal variation with nighttime values of 37¿6 pptv and average values of 87¿47 pptv. Possible origins of the diurnal variation are discussed. For such conditions, the NO mixing ratio generally was unmeasurably small, certainly less than 10 pptv. The excess NOx is also higher during the day compared with nighttime values of about 70 pptv. Further studies are required to identify the compounds involved.