[en] The interpretation of many thermalhydraulic experiments related to the Loss of Coolant Accident, shows the interest of using a six first order partial-differential equation system, to describe the thermalhydraulic behaviour of two-phase flow. Such a model needs the knowledge of mass, momentum and energy transfers across the interface between liquid and gas. It is necessary to distinguish between two kinds of transfers: the first (which produce entropy at the interface) are called irreversible; while the last don't produce interfacial entropy and are called reversible. The irreversible transfers can be formulated using the theory of irreversible thermodynamics. The reversible transfers are now unknown. However, in the hypothesis of an homogeneous equilibrium flow, the transfers are only of the reversible form. Their formulation in a six equations model is deduced from degenerative criteria. These criteria are developped. A model without reversible terms is studied. Its characteristic roots permit the definition of a Mach number and, consequently, a critical flow which depends on the local values of the flow parameters in a cross section. However, such a criterium, is inadequate for engineering computation, and for the understanding of flow blockage phenomenon. The definition of a PSI function which depends on the Mach Number and all the transfers terms permits a better approach of this phenomenon. Finally the analysis of an air-water MOBY-DICK experiment is presented illustrate this purpose