The method of solving the mixture EOS is central to the application of reactive flow in hydrodynamic simulations of explosive response. The characteristic of reactive flow models that sets them apart from lighting-time schemes is the inclusion of reactant, reacted explosive, and their mixture in the initiation and propagation process. It is important, therefore, to establish an internally self-consistent method to carry out the burning (transformation) process. A number of methods have been employed to reach local pressure equilibration of the mixed phases in reactive flow models. We will show that transformation and heat release in a fixed volume, isochoric burn, can be formulated in a manner which is internally consistent and which provides an effective method for the pressure equilibration.