The Uzon Caldera is an active As-Sb-Au epithermal system of the Kamchatka Peninsula, Far-East Russia. Using dD/d18O/d34S and dissolved ion chemistry, three thermal fluid types were distinguished at the surface as follows: 1) an acid sulfate type 2) an alkali chloride type and 3) a dilute type resulting from the mixing of alkali chloride and cold surface meteoric fluid components. Interpretations made from the presence of these three thermal fluid types were used in conjunction with the d34S of the S-bearing alteration minerals found within and around the various water and gas sources to construct a sulfur evolution model for the calderas hydrothermal system. Model results show the chemical and isotopic processes responsible for the speciation and isotopic signature of the S-bearing phases collected at the surface (both aqueous and mineral) are not only dictated by geology at depth, but are also influenced by microbial processes at the surface.