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Thermodynamic and phase diagram experimental data relevant to the Zr-O binary system have been assessed. A set of Gibbs energy functions describing the phases in this system is presented. The adjustable parameters of the models are obtained by least-squares fit to the experimental data. The liquid phase is described by Hillert's partially ionic liquid model. The interstitial solid solution model is applied to the metallic zirconium solid solutions beta Zr and alpha Zr. The ordering alpha Zr is described by a three-sublattice model with Bragg-Williams treatment of ordering. The two modifications of the zirconium dioxide, alpha ZrO2 and beta ZrO2, are modelled as stoichiometric phases. The modification gamma ZrO2 is described by the Wagner-Schottky model expressed in the compound-energy formalism. The gas phase is treated as an ideal solution of the species Zr, Zr-2 O-1, O-2, O-3, ZrO and ZrO2. The calculated phase diagrams and values for the thermodynamic properties are compared with experimental data.