Systematic investigations on the vaporization and gas phase chemistry of metal halides and their quasi-binary systems were carried out to support the development of metal halide lamps. Homo- and heterocomplexes were identified and the thermochemical data describing the stability of the complexes determined. This paper summarizes the thermochemical data on the complex stabilities. The data were used for thermochemical computations to elucidate the high temperature chemistry in metal halide lamps.