The oxidation of non-stoichiometric wustite by water delivers hydrogen. It is one step of a thermochemical metal oxide cycle for solar energy storage in chemical potential. The reaction was studied in a tubular furnace with on-line gas analysis and further characterised in detail by DTA, high-temperature X-ray powder diffraction. The influence of non-stoichiometry, morphology and temperature on the mechanism and kinetics of the water-splitting reaction was determined.