A novel method for measuring atmospheric peroxy radical concentrations has been developed and tested in various field and laboratory studies. The method relies on amplifying chemical conversion of peroxy radicals to gaseous sulfuric acid via the chain reaction with NO and SO2 (NO/SO2 chemical amplifier) and detection of the reaction product sulfuric acid by ion molecule reaction-mass spectrometry (IMR-MS). The high sensitivity of IMR-MS and the low atmospheric background of gaseous H2SO4 allows for work with small chain lengths and short reaction times, which reduces the impact of potential interference and loss processes. The basic kinetic foundations of the amplification process including its sensitivity to interference and radical loss processes are discussed. Field and laboratory measurements indicate nominal operation of the NO/SO2 chemical amplifier system and its suitability for highly sensitive peroxy radical measurements even under atmospheric conditions characterized by low peroxy radical concentrations such as nighttime, winter, and high-latitude conditions.¿ 1997 American Geophysical Union