The first selective HPLC methods for the determination of peroxyacetic acid (PAA) in gas phase samples have been developed. PAA reacts with 2-([3-{2-[4-amino-2-(methylsulfanyl)phenyl]- 1-diazenyl}phenyl]sulfonyl)-1-ethanol (ADS) to form the corresponding sulfoxide. Sampling may be performed in impingers using aqueous solutions of the reagent or by test tubes with the reagent coated on a solid sorbent. Sulfide and sulfoxide are separated by means of HPLC and detected at a wavelength of 410 nm. The method is highly selective for PAA in the presence of hydrogen peroxide when sampling in impingers. A 10,000-fold excess of hydrogen peroxide leads to the same peak area compared to PAA. Limit of detection is 10(-8) mol PAA, thus corresponding to PAA concentration of 46 ppb when using a sampling time of 10 min with a flow-rate at 500 ml/min. Another sulfide reagent, methyl-p-tolyl sulfide (MTS) has been used in a similar way with impinger sampling. Major advantages of ADS towards MTS are improved UV-Vis spectroscopic properties and reduced volatility.