We have studied the reaction of water vapor with a polycrystalline vanadium surface using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) which allows the investigation of the chemical composition of the vanadium/water vapor interface at p(H2O) in the Torr range. Water dissociation on the vanadium surface was studied under isobaric conditions at p(H2O) ranging from 0.01 to 0.50Torr and temperatures from 625K to 260K, i.e. up to a relative humidity (RH) of ~15%. Water vapor exposure leads to oxidation and hydroxylation of the vanadium foil already at a pressure of 1×10−6Torr at 300K (RH~4×10−6%). The vanadium oxide layer on the surface has a stoichiometry of V2O3. Initial adsorption of molecular water on the surface is observed at RH>0.001%. Above a RH of 0.5% the amount of adsorbed water increases markedly. Experiments at increasing temperatures show that the water adsorption process is reversible. Depth profile measurements show a thickness for the vanadium oxide layer of 3–5 mono layers (ML) and for vanadium hydroxide of 1–1.5 ML over the whole RH range in the isobar experiments. The thickness of the adsorbed water layer was found to be in the sub-ML range for the investigated RH's. Display omitted •We studied the interaction of water vapor with a polycrystalline vanadium foil.•The V-foil is easily hydroxylated and oxidized by dissociatively adsorbed water.•For the investigated relative humidities water coverage in the sub ML range was found.•Desorption experiments show that the molecular water adsorption process is reversible.