A computational study of gold(II) disproportionation is presented for the atomic ion as well as complexes with chloride and neutral ligands. The Au2+ atomic ion is stable to disproportionation, but the barrier is more than halved to 119 kcal/ mol in an aqueous environment vs 283 kcal/ mol in the gas phase. For dissociative disproportionation of chloride complexes, the loss of chlorine, either as an atom (Delta G(aq) = + 20 kcal/ mol) or as an anion (Delta G(aq) = + 15 kcal/ mol) represents the largest calculated barrier. The calculated transition state for associative disproportionation is only 9 kcal/ mol above separated (AuCl3-)-Cl-II - anions. For the disproportionation of (AuL3)-L-II complexes with neutral ligands, disproportionation is highly endergonic in the gas phase. Calculations imply that for synthesis of a monometallic Au-II complex, a nonpolar solvent is preferred. With the exception of [Au(CO)(3)](2+), disproportionation of (AuL3)-L-II complexes to (AuL)-L-I and (AuL3)-L-III is exergonic in solution phase for the ligands investigated. The driving force is provided by the very favorable solvation free energy of the trivalent gold complex. The solvation free energy contribution to the reaction (Delta G(solv)) is very large for small and polar ligands such as ammonia and water. Furthermore, calculations imply that choosing ligands that would yield neutral species upon disproportionation may provide an effective route to thwart this decomposition pathway for Au-II complexes. Likewise, bulkier ligands that yield larger, more weakly solvated complex ions would appear to be desirable.