The stability fields of binary selenide minerals calculated thermodynamically relative to native elements, sulfide, telluride, and oxide minerals as a function of temperature have been used to characterize the genesis of selenide-bearing deposits. Selenide-bearing hydrothermal deposits are restricted mainly to four major types: "telethermal" selenide vein-type deposits, unconformity-related deposits, sandstone-hosted uranium deposits, and Au-Ag epithermal volcanic-hosted deposits. The selenide minerals in telethermal selenide vein-type and selenide-bearing unconformity-related uranium deposits formed from hydrothermal fluids at conditions within the stability field of hematite, between 5.8 and 7 log units in the f (sub O2(g)) above the hematite-magnetite (HM) buffer (at 100 degrees C), and with an f (sub Se2(g)) /f (sub S2(g)) ratio higher than unity. These high f (sub O2(g)) conditions enhance the separation of selenium from sulfur, leading to a high f (sub Se2(g)) /f (sub s2(g)) ratio and deposition of a diverse group of selenides. Selenide deposition was not significantly influenced by wall-rock buffers, but late reequilibration and dissolution of some earlier formed selenides, such as krutaite (CuSe 2 ), probably occurred in some selenide-bearing unconformity-related uranium deposits. Mineral parageneses in sandstone-hosted uranium deposits suggest that they formed at higher f (sub Se2(g)) and lower f (sub O2(g)) (0-5.8 log units above the hematite-magnetite buffer at 100 degrees C) than did unconformity-related uranium deposits and selenide vein-type deposits. The selenide-bearing Au-Ag epithermal, volcanic-hosted deposits were formed from hydrothermal fluids with f (sub O2(g)) below or near the hematite-magnetite buffer and at an f (sub Se2(g)) /f (sub S2(g)) ratio lower than unity. Low f (sub Se2(g)) /f (sub S2(g)) ratios and the presence of sulfide assemblages that buffer f (sub Se2(g)) and f (sub S2(g)) prevent the enrichment of selenium in the hydrothermal fluid and deposition of selenide minerals except for silver selenides. Therefore, a selenium-rich, relatively reduced (e.g., below the hematite-magnetite buffer) hydrothermal fluid can deposit only silver selenides; no other selenide minerals can deposit from a fluid of this type. An oxidizing environment (e.g., close to the anglesite-galena buffer) is essential to form most selenide minerals. The actual mineralogy and the amount of selenide minerals that form from such an oxidizing fluid is a function of f (sub Se2(g)) and ultimately reflects the concentration of selenium in the fluid. A selenium-rich fluid alone can account for the presence of silver selenides in epithermal Au-Ag deposits, but both an oxidizing and a selenium-rich fluid is required to form the selenide mineral assemblages observed in telethermal selenide vein, unconformity-related, and sandstone-hosted uranium deposits.