Thermodynamic and phase diagram data relative to the O-Sn system have been assessed. The existence, the type and the temperature of invariant equilibria Melt1((l)) reversible arrow Melt2((l)) + SnO2(s) and Sn3O4(s) reversible arrow 2SnO((s)) + SnO2(s) have been the subject of DSC and XRD experiments. The adjustable parameters of the modelling are obtained by the CALculation of PHAse Diagram (CALPHAD) method fitted thanks to diagram and thermodynamic data through Thermo-Calc(TM) software. In particular, the liquid phase is described by the way of the Hillert's partially ionic liquid model. The final result is in good agreement with all of the experimental data available. Activities of tin and oxygen versus composition at different high temperatures can be calculated. (C) 2003 Elsevier B.V. All rights reserved.