The chlorine activation reaction ClONO2 + HCl → HNO3 + Cl2 was investigated in the laboratory on hydroxylated α-alumina surfaces under reactant partial pressure, humidity, and temperature conditions covering those which are encountered at mid-latitudes in the lower stratosphere. The measured reaction probability is γ ∼ 0.02. This result has implications with regard to the stratospheric impact of launch vehicles utilizing solid-fuel rocket motors (SRMs), such as the Space Shuttle. The exhaust from SRMs consists of alumina particles and of HCl and other vapors. The reaction probability measured in this work suggests that the ozone depletion potential of SRMs may be higher than that predicted on the basis of the chlorine emissions alone, especially at mid-latitudes in the lower stratosphere, where catalytic chlorine activation by background sulfuric acid aerosols is very inefficient. The reaction probability on Pyrex glass was found to be similar to that on α-alumina. The reaction mechanism appears to be determined by the water layers adsorbed on the surface, rather than by the detailed nature of the refractory surface itself. Copyright 1997 by the American Geophysical Union.