In our research program on the effect of interstitial gas on the handling of fine powders the influence on mixing was investigated. Two types of mixing were investigated: the mixing of two powders of which all relevant properties were equal, and the mixing of two powders with widely different properties. The experiments were performed in a rotating drum. Powder composition was established with a probe, which measured the optical reflectivity of the powder. This method proved to be fast and easy while being sufficiently accurate. It was concluded that for powders with (nearly) equal properties the speed of mixing can be increased by increasing the gas viscosity and/or gas pressure. For powders which show a segregative behavior, the final quality of the mix can, however, be improved by lowering the gas viscosity and/or gas pressure.