Hydride affinities of organic cations in solution, defined as the free energy change for the reaction (R+)s + (H-)s a (RH),, where the subscript s refers to the solvent, can be determined using either of two equations (eqs 5 and 7) derived from thermochemical cycles. The degree of uncertainty in AGhydridc(R+)s depends primarily upon the errors associated with the determination of the reversible electrode potentials for the ( R * / R -) and (R+/R') couples. In this study, AGhydride(R+)s values have been derived for a number of stable carbenium ions in dimethyl sulfoxide solution applying eq 7 with reversible electrode potentials. Values with a higher degree of uncertainty were also obtained from ring-substituted toluenes, 9-substituted fluorenes, and a-or 10-substituted 9-methylanthracenes, employing irreversible electrode potentials. The fact that the potentials for the (R*/R-) and (R+/R*) are summed in eq 7 gives rise to a partial cancellation of errors due to kinetic shifts of the experimental potentials from the reversible values. Structural effects on free energies of hydride transfer are discussed, comparisons with available gas-phase values are made, and ionic solvation energies are calculated in some cases.