Thermodynamically, aqueous polysulfide solutions are unstable, and sulfur, dissolved as polysulfide species, S-x(2-), x = 2-5, will decompose to thiosulfate, S2O32- in accord with the net consumption of dissolved sulfur : S-dissolved + OH- --> 1/4S(2)O(3)(2-) + 1/2HS(-) + 1/4H(2)O. The distribution of species, and measured and modeled rate of polysulfide decomposition, are investigated for a wide range of polysulfide solution compositions from 25 to 85 degrees C. The observed decomposition of sulfur concentration, C-s, is consistent with an activation energy for sulfur decomposition of 104(+/-5) kJ mol(-1) in accord with the following rate of sulfur loss : -dC(s)/dt = -k(f)[S-5(2-)] [OH-]/[HS-]; k(f) = 10-((10.7-5430/T(K))), where C-s is the concentration of dissolved zerovalent sulfur distributed among all polysulfide species. At temperatures up to 85 degrees C, polysulfide solutions are extremely stable under conditions of high polysulfide concentration (>5 m K2S4) and low KOH concentrations. Under these conditions, zerovalent sulfur dissolved in these solutions should be stable on the order of years.