The physics of the two-phase flow models involved on fast transient phenomena has a predominant importance in nuclear studies and in other industrial fields, where fluid-structure interaction occurs. Focusing only in the aspect concerning the fluid, in this work we give some comparisons between two-phase flow models for fast transients, in order to understand the main theoretical differences and the related physical aspects due to the different thermodynamic non-equilibria involved. In the presented models, an EoS as a function of specific volume (v) and specific internal energy (e) of the fluid is always needed in order to close the system of the governing flow equations, based on a conservation law.