In this paper deaeration efficiency is studied and discussed with regard to a seawater desalination plant under construction in Ruwais (United Arab Emirates) for ADNOC (Abu Dhabi National Oil Company). In particular, a modelling activity has been performed in order to optimise deareator design. Seawater flashing and stripping have been investigated, and a simulation code has been set up taking account of mass, energy and momentum balances together with phase equilibrium conditions. Parametric studies have been carried out analysing the effects of seawater and stripping steam flow rates, as well as those of temperature, pressure, packing height and other operating conditions or deaerator geometrical characteristics. Different models have been discussed interpreting flash and stripping phenomena; and in the light of the results obtained, here shown and discussed, new solutions have been proposed to reduce the costs of the deaerator installation and management.