Ab initio calculations at the MP2/6-31G(d) level of theory have been performed to determine the geometries and relative energies of many isomers of protonated octane (also known as octonium or octanium ions). Five of the 18 structural isomers of octane were considered for protonation, as they provided C-C bonds containing all possible combinations of carbon substitution types (quaternary-tertiary, quaternary-secondary, etc.). All resulting isomers Of C8H19+ feature either a CHC or a CHH 3-center-2-electron (3c2e) bond, although barrierless dissociation into an ion-molecule complex was very common. Octonium ion properties such as relative energies, 3c2e bond geometries, Mulliken partial charges, and the frequency of the most intense infrared absorption, have also been calculated. Each property is correlated to the level of substitution of C atoms in the 3c2e bond. The proton affinities of individual bonds in octane range from 154 to 187 kcal mol(-1) for C-C bonds and 139 to 150 kcal mol(-1) for C-H bonds. Alkanium (carbonium) ions of greater than four carbons have never before been studied in this depth.