This paper is aimed to show overwhelming experimental and theoretical evidence supporting the existence of organic oxonium ions (ROH2+) as mechanistic intermediates in water solution, which should be taken into account when describing important reactions, like hydrations of carbocations or C-O cleavages under acidic conditions. For the hydration reaction of tert-butyl (t-Bu+) cation, we have calculated the reaction rate between the intermediate hydrated cation t-BuOH2+ and water, showing that the concerted proton/electron transfer reaction (CPET) is very slow in comparison with experimental data. Much better accordance is achieved by assuming a sequential electron transfer/proton transfer reaction (ETPT). Thus, there is an excellent accordance between the calculated relaxation time (tau = 2.14 ps) for the ETPT process in water and experimentally determined tau values (1.0-1.5 ps) for related reactions. Moreover, there is also an excellent agreement between the potential energy of activation (del V-TS(not equal) = 3.17 kcal/mol) for the proton transfer in gas-phase, computed with the B3LYP/6-31G(d) method following the variational transition state theory (VTST), with the analogous del V(Q(EQ))(not equal) value (3.09 kcal/mol), calculated using methods based on single electron transfer (SET) reactions.