MFIX (Multiphase Flow with Interphase eXchanges) is a general-purpose computer code developed at the National Energy Technology Laboratory (NETL) for describing the hydrodynamics, heat transfer and chemical reactions in fluid-solids systems. It has been used for describing bubbling and circulating fluidized beds and spouted beds. MFIX calculations give transient data on the three-dimensional distribution of pressure, velocity, temperature, and species mass fractions.