Published thermodynamic properties of selenide minerals were used to construct fugacity-temperature diagrams to estimate relative stabilities of the minerals as a function of temperature. The phase relations among selenides, sulfides, tellurides, and oxides were investigated by constructing fugacity-fugacity diagrams at temperatures between 100 degrees and 300 degrees C. These diagrams are used to predict equilibrium mineral assemblages and their variation with temperature, the relative tendency of various metals to form selenides, and to estimate the prevailing f (sub Se (sub 2(g)) ) , f (sub S (sub 2(g)) ) , f (sub Te (sub (g)) ) and f (sub O (sub 2(g)) ) , and their evolution during mineral deposition and/or reequilibration from mineral assemblages recognized in the deposit. These diagrams are also used to explain the occurrence of selenide minerals in ore deposits and to predict the mineral assemblages of some economically important chemical elements including Au, Ag, and Cu. The stability fields of selenide minerals are generally more restricted than those of sulfide minerals but are larger than those of corresponding telluride minerals for similar values of f (sub S (sub 2(g)) ) , f (sub Te (sub (g)) ) , and f (sub Se (sub 2(g)) ) . Exceptions include hessire (Ag 2 Te) and calaverite (AuTe 2 ), whose stability fields are larger than those of naumannite (Ag 2 Se) and AuSe, respectively. Most oxide minerals, except cassiterite, magnetite, hematite, and uraninite are unstable relative to the corresponding selenide minerals in typical hydrothermal fluids. The larger the stability field of the selenide, the more common it is in ore deposits (e.g., clausthalite, PbSe). Some transition metal selenide minerals that have not been found in nature (e.g., Ni 3 Se 2 are stable only at very low f (sub S (sub 2(g)) ) and/or f (sub O (sub 2(g)) ) and are unlikely to occur in the earth's crust. Some compounds such as AuSe and those in the Tl-Se binary system are likely to be found in natural assemblages. They have not been observed because of our failure to detect them or because ternary gold and thallium minerals, respectively, are formed instead. The electrum-naumannite assemblage may act as a sliding-scale indicator of f (sub Se (sub 2(g)) ) , similar to the electrum-Ag 2 S system for f (sub S (sub 2(g)) ) .