Rare earth metal halides in combination with alkali metal halides such as NaX-DyX3 (X = Cl, Br, I) are used as part of the fill chemistry in high intensity discharge (HID) light sources. Thermodynamics descriptions of these metal halide melts are necessary to predict vapor compositions. While there are thermodynamic data available for the chloride and iodide based systems, there are no reported thermodynamic measurements and no published phase diagrams for the bromide system, Therefore, we have estimated the thermodynamic properties for the solid, liquid and gaseous NaBr-DyBr3 system and calculated the gas phase concentrations of the various species present above melts with varying composition. In order to accomplish this, we have relied upon the published phase diagrams of the NaCl-DyCl3 and NaI-DyI3 systems in addition to the reported enthalpies of mixing of the chloride system. Using a multi-species melt model (MSMM), we have found that we can get a reasonable phase diagram of the NaBr-DyBr3 system together with reasonable enthalpies of mixing for all three systems.