The potential energy surfaces for the reaction between H2O and the protonated alcohols MeOH2+, EtOH2+, PriOH2+, and ButOH2+ have been explored by means of high level ab initio theoretical methods. Both nucleophilic substitution (SN2) and elimination (E2) pathways have been investigated. Front side (SNF) and the familiar back side (SNB) Walden inversion attack of the nucleophile have been found to be competing for the H2O–ButOH2+ system. In contradiction with the customary relationship between so-called “steric effects” and barrier heights—more alkyl-substituted SN2 reaction centres have higher SN2 reaction barriers—the SN2 reaction barriers are found to be Et > Me > Pri > But. This result is in excellent agreement with available experimental data.