Due to the toxicity of selenium it is quite essential to analyze the conditions under which selenites and selenates replace selenides in the oxidation zones of sulfide ore deposits and upon weathering of technologic waste. This purpose is performed by summarizing in Eh-pH diagrams the results of calculations of equilibrium, and considering geologic occurrences. © V.G. Krivovichev, M.V. Charykova, 2006.