Thiocyanate has been identified and studied as a promising alternative lixiviant for gold in acidic solutions. Eh-pH and ion species distribution diagrams for SCN-H2O, Au-SCN-H2O, Ag-SCN-H2O, Cu-SCN-H2O, and Fe-SCN-H2O systems were constructed to predict the behavior of each metal ion in the thiocyanate system and also to explain the experimental results. Thermodynamic analyses suggest that gold can be leached by thiocyanate under appropriate leaching potentials, forming aurous or auric complexes with thiocyanate, depending on the thiocyanate concentration and leaching potential. According to species distribution diagrams, silver (I) and copper (I) form insoluble salts at moderate thiocyanate concentrations and are soluble at low and high thiocyanate concentrations. Ferric ion forms a series of complexes with thiocyanate. The study of the ferric ion effect indicates that gold can be leached in acid thiocyanate solution with ferric sulfate as the oxidant. Also the presence of excess ferric ion reduces the apparent thiocyanate activity for copper (I) and silver (I) dissolution. The findings of this thermodynamic assessment are useful in the analysis of some of the phenomena encountered in the leaching and recovery of gold from thiocyanate solutions as discussed in subsequent papers.