A simplified approach is described for selection of the constitutive relations for the interphase exchange terms in the two-fluid code, THERMIT, The approach used distinguishes between pre- and post-critical heat flux conditions. Interfacial mass, energy, and momentum exchange terms are selected and tested against one-dimensional measurements for a wide range of mass flow rate, pressure, and void fraction conditions. It is concluded that the simplified regime-map approach leads to accurate predictions for light water reactor applications, excluding depressurization events.