Metal halide lamps are very efficient light sources based on a Hg plasma arc with metal halide salt additions. In spite of their high efficiency, the lamps suffer from several problems, such as color separation and instabilities, which currently hinder large-scale use. These phenomena are caused by a complex interaction of convection and diffusion flows in the plasma. In order to unravel the various contributions, experiments under microgravity have been performed where convection is absent.The experiments confirm the previously held qualitative views, but also provide absolute data on densities and temperatures that will be used to validate numerical models of these lamps.Keywords: plasmas; lamps; metal halide; microgravity; high pressure; lighting.*Paper presented at the 17th International Symposium on Plasma Chemistry (ISPC 17), Toronto, Ontario, Canada, 7-12 August 2005. Other presentations are published in this issue, pp. 1093-1298.