A computational model capable of capturing fully compressible multiphase flow including energy conservation is presented. The model is a finite volume form based on the Reynolds Averaged Navier-Stokes Equations and is capable of considering fully general equations of state. The preconditioning matrix is presented and ensures a well conditioned eigensystem, essential for efficient and accurate multiphase computations. Solutions are given representing both the utility of the preconditioning method as well as the ability of the model to capture highly compressible multiphase flow fields. When considering compressible flows where thermal effects are significant and a general equation of state is necessary, energy conservation in a multiphase flow field is shown to be a requirement.