NO_x has been measured in remote marine air via chemiluminescence detection using two different methods for NO₂ to NO photolytic conversion: (a) internal diodes and a reaction chamber made of Teflon-like barium-doped material, which causes a NO₂ artefact, and (b) external diodes and a quartz photolysis cell. Once corrections are made for the artefact of (a), the two converters are shown to give comparable NO₂ mixing ratios, giving confidence in the quantitative measurement of NO_x at low levels.