In order to evaluate the thermodynamics and reaction mechanisms of the adsorption and reduction of gold on galena surfaces, reaction energies were calculated for the adsorption of AuCl4- from solution and for the stepwise reduction of gold from Au(III) to Au(0) by using an ab-initio cluster approach. The adsorption of gold chloride onto galena and the reduction from Au(III) to Au(0) were found to be slightly exothermic, but only if hydration energies are included. All adsorbate structures with intermediate oxidation states occupy local energy minima, that is, they are potentially metastable. Even though gold is in the formal oxidation state Au(III) in galena-AuCl3, the main electron transfer from the galena surface to the gold atom has already taken place by withdrawing about 0.1 unit charges from all sulfur atoms in the cluster. This mechanism explains the unique affinity of gold to semiconducting sulfide surfaces. Au(0) is stabilized by the formation of gold dimers and islands due to the formation of gold-gold metal bonds.