Based on first-principles density functional theory and finite temperature molecular dynamics calculations, we predict that H 2O can be dissociated into its constituents O and H at specific vacancy defects of single-layer MoS 2 honeycomb structure, which subsequently are bound to fourfolded Mo and twofolded S atoms surrounding the vacancy, respectively. This exothermic and spontaneous process occurs, since the electronegativity and ionization energy of Mo are smaller than those of H. Once desorbed from twofolded S atoms, H atoms migrate readily on the MoS 2 surface and eventually form free H 2 molecules to be released from the surface. Present results are critical for acquiring clean and sustainable energy from hydrogen. © 2012 American Physical Society.