Wet air oxidation process is used to treat wastewater with high chemical oxygen demand. Temperatures up to 350°C and pressures up to 30 MPa, are operated to develop a non-catalytic process. In the literature, there is some data about the phase equilibrium of water-air system but on larger domains and not accurate enough in our range. To study the process, it is necessary to complete the data with adapted pressures, temperatures and compositions. An experimental set-up has been developed to realise equilibrium measurements as well as trials on waste degradation. It consists of a stirred reactor of 150 mL equipped with two sapphire windows (diameter 4.1 cm) allowing a total visualization of the reactive chamber. The results of experiments give the volume occupied by the liquid and the gas phases, for a combination of temperature, pressure and global composition. Measurements are realised to establish dew curves for water-air, water-nitrogen and water-waste-nitrogen systems. Nitrogen has the same behaviour as oxygen and can replace it in mixtures for the phases equilibrium studies with the waste to avoid any reaction. The presence of the waste, in the concentrations studied, has no significant influence on the equilibrium points. Those data serve to model the liquid-vapour equilibrium in the appropriate domain first with an ideal model and then with equation of state Soave-Redlich-Kwong and appropriate mixing rules. Those equilibrium models are then included in commercial engineering software to simulate WAO processes. With the objective to develop a process energetically optimised, the second part of this work is dedicated to perform energy balances for each component and for the whole process