Room-temperature reactions of the atomic cations Sr + and Rb + have been surveyed systematically with a variety of gases using an Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and product distributions have been measured in He buffer gas at 0.35Torr and 295K for reactions of Sr + and Rb + with CH 3 F, CH 3 Cl, N 2 O, CO 2 , CS 2 , SF 6 , D 2 O and NH 3 . Rb + (s 0 ) is seen to be quite inert with these molecules and reacts either slowly by molecule addition or not at all, while Sr + (s 1 ) is much more reactive with all these 8 molecules, especially with CH 3 F, CH 3 Cl, N 2 O and SF 6 . Sr + reacts with CH 3 F and SF 6 by F-atom transfer, with CH 3 Cl by Cl-atom transfer and with N 2 O by O-atom transfer, and the reaction rate coefficients are all quite high, k≥1.4×10 −11 cm 3 molecules −1 s −1 . The extreme differences in reactivity with CH 3 F, SF 6 , CH 3 Cl and N 2 O provide a chemical basis for the separation of isobaric interferences of 87 Rb + and 87 Sr + often encountered in ICP–MS. Among these four molecules, SF 6 exhibits the largest difference in reactivity, almost a factor of 10 4 , and so is identified as the kinetically recommended reagent for the chemical resolution of the isobaric interference of 87 Rb + and 87 Sr + .