Quantum chemistry calculations have been used to study the uncatalyzed transfer hydrogenation between a range of hydrogen donors and acceptors, in the gas phase and in solution. Our study shows in the first place that in order to obtain reliable condensed-phase transition structures, it is necessary to perform geometry optimization in the presence of a continuum. In addition, the use of a free energy of solvation obtained with the UB3-LYP/6-31+G(d,p)/IEF-PCM/UA0 combination, in conjunction with UMPWB1K/6-311+G(3df,2p)//B3-LYP/6-31+G(d,p) gas-phase energies, gives the best agreement with experimental barriers. In condensed phases, the geometries and energies of the transition structures are found to relate to one another in a manner consistent with the Hammond postulate. There is also a correlation between the barriers and the energies of the radical intermediates in accord with the Bell-Evans-Polanyi principle. We find that in the gas phase, all the transfer-hydrogenation reactions examined proceed via a radical pathway. In condensed phases, some of the reactions follow a radical mechanism regardless of the solvent. However, for some reactions there is a change from a radical mechanism to an ionic mechanism as the solvent becomes more polar. Our calculations indicate that the detection of radical adducts by EPR does not necessarily indicate a predominant radical mechanism, because of the possibility of a concurrent ionic reaction. We also find that the transition structures for these reactions do not necessarily have a strong resemblance to the intermediates, and therefore one should be cautious in utilizing the influence of polar effects on the rate of reaction as a means of determining the mechanism.