A selected ion flow drift tube (SIFDT) apparatus and a proton transfer reaction mass spectrometer (PTR-MS) were used to study the reactions of protonated water H3O+ and the proton bound water dimer (H2O)2H+ with PAN (peroxyacetic nitric anhydride, CH3C(O)OONO2), PPN (peroxypropionic nitric anhydride, CH3CH2C(O)OONO2) and MPAN (peroxymethacrylic nitric anhydride, CH2C(CH3)C(O)OONO2). The protonated hydrates react rapidly with PAN, PPN and MPAN. Product ion peaks using ambient air as reactant gas could be unequivocally attributed to PAN, PPN and MPAN utilizing the thermal decomposition of these compounds on a hot surface. The results demonstrate that (H2O)nH+ (n=1,2) ions are excellent reagent ions for the real-time detection of PAN and other organic nitrates in ambient air. ¿ 2000 American Geophysical Union