The use of the one-dimensional two-phase flow six-equation model requires knowledge of mass, momentum, and energy transfers between the phases. These transfers can be expressed from the flow parameters and their derivatives. The first part of this paper is devoted to the formulation of the entropy production at the interface as a function of the velocity, Gibbs potential and temperature of each phase. It is assumed that each transfer can be expressed in the form where R is the reversible part and δR the irreversible part of the transfer R. The linear theory of irreversible thermodynamics allows the formulation of δR. The expression of R may include differential terms. In the second part of this paper, we show how to write interfacial transfer terms to reduce the six-equation model into a lower order model. The last part of this paper presents an original method for computing critical flow, taking into account the flow blockage phenomenon, which is observed when variations of downstream conditions do not produce any significant effect on the upstream flow, even though the fluid velocity is less than the sound velocity.