Gas-phase ion molecule reactions of the primary atmospheric cations (NO+, O-2(+), O+, N+, and N-2(+)) With two isomers of octane, n-C8H18 and iso-C8H18 (2,2,4-trimethylpentane) have been studied using a variable temperature selected ion flow tube instrument. Reaction rate constants and product branching fractions were measured from 300 to 500 K. The reactions of O-2(+), O+, N+, and N-2(+) with n-C8H18 and iso-C8H18 proceed at the collision rate via dissociative and/or nondissociative charge transfer. The NO+ reactions occur primarily by hydride transfer; the reaction rate for the straight-chain isomer is only one-fourth the collision rate, while the reaction rate for the branched isomer is significantly enhanced. The reaction of n-octane with each atmospheric cation generates two to four major product ions and numerous minor species. The major ionic products are alkyl cations, CnH2n+1+, where the degree of fragmentation of the hydrocarbon chain is governed largely by the reactant ion recombination energy. The largest ionic products observed, n-C8H17+ and n-C8H18+ thermally decompose at temperatures above 300 K. The reaction of isooctane with each atmospheric cation generates fewer minor species than was observed in the n-octane reactions, and only one to three major product ions are observed. The main ionic product of the O-2(+), O+, N+, and N-2(+) reactions is the alkyl cation C4H9+, although several reactions also produce significant amounts of the related radical cation C4H8+. The main ionic product of the NO+ reaction, C8H17+, thermally decomposes into C4H9+ at temperatures above 300 K. Presumably, the specificity of the product ion formation in the iso-C8H18 reactions is a consequence of the neutral reactant structure. Except for thermal fragmentation of C8Hm+ product ions at temperatures above 300 K, there is little temperature dependence on the product ion branching fractions for the reported reactions.