The study of phase equilibria involves the determination of physical and chemical relationships among either natural or synthetic minerals. Although there are diverse hydrothermal ore deposits with various type of ore minerals, sulfides constitute the most important and dominant group of ore minerals that are mined from the Earth’s crust. Besides the choice of sulfide systems is because of their obvious importance in diverse types of natural ore mineral assemblage as components of many larger ore-forming systems (Vaughan & Craig, 1997). In general, the parameters considered in our discussion include intensive thermodynamic variables such as pressure, temperature, composition (Xi) and fi or ai (fugacity or activity of ambient fluid species) and to represent them in the form of P–T, T–Xi, \( \mathrm{T}\hbox{--} {\mathrm{f}}_{{\mathrm{S}}_2} \) , \( \mathrm{T}\hbox{--} {\mathrm{f}}_{{\mathrm{O}}_2} \) , \( {\mathrm{f}}_{{\mathrm{S}}_2} \) – \( {\mathrm{f}}_{{\mathrm{O}}_2} \) diagrams. Obviously, the aim of any phase equilibrium study is to simulate the natural ore-forming environments in the laboratory. But such crucible–to nature extrapolation is often complicated and hindered by complexity of natural ore systems, due the ‘plague of metastability’. Nevertheless, we will demonstrate how the results of sulfide phase equilibrium experiments help to understand ore-forming processes by the study of ore textures. Application of laboratory data to natural assemblages for quantitative interpretation in the form of phase diagrams requires recognition of independent variables. As discussed in the previous chapter, the above goal is achieved through the Gibbs phase rule, i.e., a fundamental statement in geochemical thermodynamics. In this chapter, we present phase equilibrium and thermodynamic aspects of some common sulfide systems. We exclude the sulfosalt-bearing systems, which were extensively studied lately.