The mechanism of the bimolecular reactions of ethyl cation with ethane and propane and of s-propyl cation with ethane propane, and isopentane is theoretically investigated by means of the B3PW91 density functional method. The study includes complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved, calculation of the reaction enthalpies and activation energies for the different elemental steps, and obtainment of the equilibrium constants and relative reaction rate constants by means of transition state theory. It is found that the interaction of a carbenium ion with a saturated alkane always results in formation of a stable carbonium ion intermediate and that different intramolecular rearrangements of this common intermediate can explain the mechanism of acid-catalyzed hydrocarbon reactions, such as hydride transfer, disproportionation, dehydrogenation, and alkylation.