In the gas phase O2+ reacts with methane at 300 K to produce a hydrogen atom and the CH3O2+ ion. The structure of this ion has recently been determined to be H2COOH +, methylene hydroperoxide ion. The reaction rate coefficients and product distributions have now been measured at 300 K for the CH nD4-n isotopes. The reaction shows both inter- and intramolecular isotope effects, e.g., CH2D2 reacts more slowly than methane and more rapidly than CD4, but loses hydrogen or deuterium with equal probability. The ion readily transfers HO+ to alkenes, CS2, and many other neutral molecules. The reaction with CS2 has been used to investigate the isotopic distribution within mixed isotope product ions. In addition, the reaction rate coefficients for both CH4 and CD4 have been measured as functions of temperature between 20 and 500 K; in both cases a clear minimum is observed in the reaction rate coefficient near room temperature. A mechanism for the reaction is proposed which allows us to model the temperature dependence of the reaction rate coefficient over the entire range for which there are data.