We report a first principles molecular dynamics study on hydration and acidity of polysulfide species. Hydration structures of (poly)sulfides species are pictured by analyzing the trajectories. With the vertical energy gap technique, our calculations reproduce the acidity constants of H2S and predict chain length independent acidity for polysulfanes: pKa(1)s are about 7.0 and pKa(2)s are around 9.0. This indicates H2Sn, HSn and S-n(2) can all be dominant under common pH. Furthermore, our simulations show that stable Au+-HSn complexes can form in solutions, which implies that polysulfides can perform as metal complexing agents in hydrothermal processes. (C) 2013 Elsevier B. V. All rights reserved.