Rate coefficients for reactions of the ions 0 + , CO + , and CH + with atomic hydrogen have been measured for the first time at 300 K. This provides basic data for the ion chemistry of planetary atmospheres, cometary atmospheres, and interstellar molecular clouds. The 0 + + H measurement supports quantal calculations of this reaction. The CO + +H reaction provides an example of partial spin nonconservation in a charge-transfer reaction occurring in a deep potential well. Reactions of the same ions with H 2 that have been measured elsewhere are also reported.PACS numbers: 82.30.Fi, 82.40.We, 98.40.C + Reactions of ions with atomic hydrogen may be important in interstellar molecular clouds and in planetary and cometary atmospheres where atomic hydrogen is abundant. They may also be important in laboratory discharges, plasmas, and combustion systems. Atomic hydrogen reactions are particularly important at the boundaries of fusion reactors. The relative simplicity of the atomic hydrogen reactant allows theoretical determination of rate coefficients in some cases. For these reasons we have measured the previously unmeasured reactions of 0 + , CO + , and CH + ions with atomic hydrogen. Measurements with molecular hydrogen, which have previously been carried out, were also a consequence of the experiment.The measurements were carried out at 300 K in selected-ion-flow drift tubes in Trento and in Innsbruck. The measurements involving H 2 as a reactant were straightforward, just as described by McFarland et al. l For the atomic hydrogen reactions, H 2 was dissociated in a high-frequency discharge (Trento) and, alternatively, by surface dissociation (Innsbruck) and the mixture of H 2 and H introduced into the drift tube. The problem of competition between the reactions of the above ions with H and with H 2 was solved in two steps. (a) The degree of dissociation of hydrogen was determined in each case by applying a recently developed method 2 involving the reaction of C0 2 + with H and H 2 . The C0 2 + reaction with H 2 produces C0 2 H + +H but no COH + product ions, while C0 2 + reacting with H results in COH + product ions but no C0 2 H + . The known rate coefficients for these reactions allow us to determine the degree of dissociation from the differences in the respective C0 2 + ion decreases and C0 2 H + ion increases when the dissociation source is on and off. (b) With knowledge of the degree of hydrogen dissociation, the same method 2 is then applied (observation of the decrease of the ion signals of 0 + , CO + , and CH + with the source on and off) to obtain the rate coefficients for the reactions of the above ions with H 2 and H, respectively. The values obtained for the rate coefficients were the same with the two atomic hydrogen sources used and the accuracy of measurement is believed to be better than ±30%.The results are given in Table I. For the reaction 0 + ( 4 S)+H( 2 S)