Measurements of atmospheric OH concentrations were conducted between August 2010 and July 2011 at Mace Head showing maximum daytime values of 0.21 (±0.25) × 106 cm−3 in winter and 2.26 (±1.37) × 106 cm−3 in summer. Plots of OH versus ozone photolysis frequency, J(O1D), exhibited strong linear correlations with slopes of 1.06 (±0.05) × 1011 cm−3 s (R = 0.75) in clean marine air and 1.31 (±0.04) × 1011 cm−3 s (R = 0.79) in mixed marine/continental air. Surprisingly, no significant difference in the former correlation was found between low and high tide periods. NO and NO2 levels in air from the marine sector (190–300°) were typically below the detection limit (30 pptv and <200 pptv, respectively). In the land sector, NO mixing ratios <50 pptv dominated most of the time, suggesting that the atmospheric oxidation efficiency in this region is predominantly characterized by primary OH sources in a low NOx environment.