This paper deals with the expansion of supercritical water at the output of the reactor of a supercritical water oxidation process. Inside such reactors, a common value of the working pressure is approximately 25 MPa. The fluid exhausting the reactor mainly contains water and carbon dioxide and salts. The operating pressure needs to be dropped in order to separate the different constituent of the fluid and to send back to environment the clean effluents. The behaviour of the salts held within the supercriticall water is very specific. Indeed, at high pressure, the salts are present at the solid state, but once some liquid water appears, a part of the salts is dissolved into this liquid. Thus, in order to be separated by classical gas/solid separation at moderate pressure, the process stream needs to be carefully expanded. This paper presents a mathematical model that has been build in order to design a capillary device for this expansion. The governing equations of the model are presented as well as the way it has been solved. Some computations around a nominal operating conditions point are performed. These computations have led us to the design of an experimental device. (C) 2006 Elsevier B.V. All rights reserved.