Abstract The on‐line detection of gaseous peroxyacetyl nitrate (PAN) using selected ion flow tube mass spectrometry (SIFT‐MS) has been investigated using a synthetic sample of PAN in air at a humidity of ∼30%. Using the H 3 O + reagent ion, signals due to PAN at m/z 122, 77 and 95 have been identified. These correspond to protonated PAN, protonated peractetic acid and its water cluster, respectively. These products and their energetics have been probed through quantum mechanical calculations. The rate coefficient of H 3 O + has been estimated to be 4.5 × 10 −9 cm 3 s −1 , leading to a PAN sensitivity of 138 cps/ppbv. This gives a limit of detection of 20 pptv in 10 s using the [M+H] + ion of PAN at m/z 122. Copyright © 2010 John Wiley & Sons, Ltd.