Consistent thermochemical data are of major importance for predicting and rationalizing stability and reactivity throughout chemistry. The free energy of hydration (Delta G(hyd)) substantially defines the aqueous chemistry of metal ions and aids our understanding of the properties of water and has thus been widely studied both theoretically and experimentally. This paper first shows that the experimental standard half reduction potential for the process Mn+ + ne(-) -> M is accurately described using a simplified version of Trasatti's thermochemical cycle involving the ionization potentials, Delta G(hyd) of Mn+, and the standard heat of vaporization (Delta H-vap) of M. This approximation, which neglects entropy, is shown to be valid both by actual performance (uncertainty similar to 0.1 V, R-2 similar to 0.99-1.00 for available data for M3+ and M2+ ions) and by application of Trouton's rule for entropies of vaporization. Second, application of the formula allows the identification of many Delta G(hyd) values not reported before. Together with previously determined values, the compiled lists of Delta G(hyd) are the most complete so far reported and are all thermochemically consistent; i.e., they agree with their corresponding thermochemical cycles. The numbers use the convention Delta G(hyd)(H+) =-1100 kJ/mol and SHE = +4.44 V but can be easily adjusted to other reference states as appropriate. Some of the new Delta G(hyd) values established here are for the catalytically important d-transition metal ions Rh3+, Re-3+,Re- Ir3+, Mo3+, W3+, Tc2+, Nb3+, Ta3+, 00+, and Ru2+. Some, such as Ir3+, are among the most inert aqua ions known. The AGhyd values have an accuracy of similar to 10 kJ/mol and are recommended for use in thermochemical calculations, for interpretation of the aqueous chemistry of the metal ions, and as benchmarks for theoretical chemistry.