Rationale. Data are required for SIFT-MS analysis of per- and poly-fluoroalkyl substances (PFAS) that are persistent in the environment and cause adverse health effects. Specifically, the rate coefficients and product ion branching ratios of the reactions of H ₃O ⁺, NO ⁺, O ₂ ^+•, O ^-•, OH ^-, O ₂ ^-•, NO ₂ ^-, and NO ₃ ^- with PFAS vapours are needed. Methods The dual polarity SIFT-MS instrument (Voice200) was used to generate these 8 reagent ions and inject them into the flow tube with N ₂ carrier gas at a temperature of 393 K. Vapours of pentafluoropropionic acid, heptafluorobutyric acid, nonafluoro-1-hexanol, perfluoro-2-methyl-2-pentene, perfluorohexanoic acid, perfluoro(2-methyl-3-oxahexanoic) acid, tridecafluoro-1-octanol, and nonafluorobutane-1-sulfonic acid were introduced in dry and humid air. Full-scan mass spectra were collected for all reagents at variable PFAS concentrations and analysed numerically. Results Rate coefficients were determined for 64 reactions, for which 55 positive and 71 negative product ions were identified. The branching ratios for the primary reaction channels were extracted from the data, and the secondary chemistry with H ₂O molecules was qualitatively assessed. The thermochemical data were calculated for the H ₃O ⁺ reactions using Density Functional Theory (DFT). Conclusions An important observation is that secondary reactions with water molecules remove the positive product ions, making them unsuitable for practical SIFT-MS analysis of PFAS vapours. In contrast, most negative reaction product ions are not significantly affected by humidity and are thus preferred for the SIFT-MS analyses of PFAS substances in various gaseous matrices.