The adsorption of water on Fe 3O 4(111) films grown epitaxially onto Pt(111) was investigated by ultraviolet photoelectron spectroscopy (UPS) in adsorption-desorption equilibrium and by thermal desorption spectroscopy (TDS). With increasing coverage both methods reveal the existence of three species on the surface: dissociatively chemisorbed water (γ), physisorbed monomeric water (β) and hydrogen bonded condensed ice (α). The corresponding isosteric heats of adsorption qst and desorption energies Edes were determined by UPS and TDS, respectively. For the α- and β-species they compare well, for the γ-species Edes is higher, indicating an activation barrier for the dissociative adsorption, which is also supported by the observed slow adsorption kinetics. The dissociation is assumed to occur at iron cations with neighboring oxygen anions acting as proton acceptors.