The use of microbalances to study gas-solid reactions at high temps. and normal pressure is often limited by fluctuations due to thermal convection in the reaction zone. The fluctuations depend on the pressure, the type of gas, the temp., and the geometry of the arrangement. The problems were overcome by distributing the gas on the circumference of the tube surrounding the specimen. The obsd. fluctuations under normal pressure (1.1 bar) and temps. ?200 Deg were <0.5 mg.