The redox stability of gold halide complexes in aqueous solution has been examined quantum-chemically by a systematic comparison of scalar- and nonrelativistic pseudopotential calculations, using both COSMO and D-COSMO-RS solvent models for water. After a computational benchmarking of density-functional methods against CCSD(T) results for the gas phase decomposition AuX4- -> AuX2- + X-2, B3LYP calculations have been used to establish solvent contributions. While relativity clearly enhances the stability of AuX4- (X = F, Cl, Br, I) complexes against X-2 elimination, solvation favors the lower oxidation state. Solvation and relativity are nonadditive, due to the relativistic reduction of bond polarity. At scalar relativistic D-COSMO-RS level, the reaction AuX4- (sic) AuX2- + X-2 is computed to be endergonic, except for X = I, where it is slightly exergonie. Under the chosen conditions, partial hydrolysis of AuCl4- to AuCl3OH- is exergonic. The latter complex in turn is stable against Cl-2 elimination. The disproportionation 3 AuCl2- (sic) AuCl4- + 2 Au-(s) + 2 Cl- is clearly exergonic. All of the computed reaction energies at scalar relativistic D-COSMO-RS level agree well with the observed speciation in dilute pH-neutral solutions at ambient temperatures.