We report laboratory studies of the detection scheme employed for in situ measurement of NOy in the atmosphere. In this technique, an air stream is passed over a hot metal (usually 24 karat (k) Au) catalyst in the presence of a reducing agent (usually CO), which converts the NOy compounds to NO. Using the NOy species NO, NO2, HNO3, and isopropyl nitrate and the potential interferences HCN, CH3CN, NH3, and N2O, we investigated: (1) conversion efficiencies as a function of pressure and catalyst temperature; (2) conversion efficiencies as a function of reducing-agent concentration with both H2 and CO; (3) the effect of humidity and O3 on conversion efficiency; (4) loss of NO in the catalyst; and (5) the efficacy and suitability as catalytic converters (or inlets) of several metals (24 k Au, 18 k Au, Au with 1% Co, Ag, Pt, stainless steel) and quartz. The most significant results are the discovery of a gas-phase process that contributes to the conversion of HNO3 to NO and the identification of conditions under which HCN, CH3CN, and NH3 are converted to NO with high efficiency. We discuss the implications of these results for in situ measurement of atmospheric NOy.¿ 1997 American Geophysical Union