Dimethyldisulfide (I) is the simplest model of the biologically relevant family of disubstituted disulfides. The experimental study of its gas-phase protonation has provided, we believe for the first time, a precise value of its gas-phase basicity. This value agrees within 1 kJ mol-1 with the results of G3 calculations. Also obtained for the first time was the reaction rate constant for the bimolecular reaction between I and its protonated form, IH+, to yield methanethiol and a dimethyldithiosulfonium ion. This constant is of the order of magnitude of the collision limit. A computational mechanistic study based on the energetic profile of the reaction, completed with Fukui's and Bader's treatments of the reactants and transition states fully rationalizes the regioselectivity of the reaction as well as the existence of a shallow, flat Gibbs energy surface for the reaction. The mechanistic relevance of the chalcogen-chalcogen interaction and the C--H...S bonds has been demonstrated.