The heterogeneous reaction of ClONO2 + NaCl --> Cl-2 + NaNO3 (eq 1) was investigated over a temperature range 220-300 K in a now-tube reactor interfaced with a differentially pumped quadrupole mass spectrometer. Partial pressures of ClONO2 in the range 10(-8)-10(-5) Torr were used. Granule sizes and surface roughness of the NaCl substrates were determined by using a scanning electron microscope, and in separate experiments, surface areas of the substrates were measured by using BET analysis of gas-adsorption isotherms. For dry NaCl substrates, both the decay rates of ClONO2 and the growth rates of Cl-2 were used to obtain reaction probabilities, gamma(1) = (4.6 +/- 3.0) x 10(-3) at 296 K and (6.7 +/- 3.2) x 10(-3) at 225 K, after considering the internal surface area. The error bars represent 1 standard deviation. The Ch yield based on the ClONO2 reacted was measured to be 1.0 +/- 0.2. In order to mimic the conditions encountered in the lower stratosphere, the effect of water vapor pressures between 5 x 10(-5) and 3 x 10(-4) Torr on reaction 1 was also studied. With added H2O, reaction probabilities, gamma(1) = (4.1 +/- 2.1) x 10(-3) at 296 K and (4.7 +/- 2.9) x 10(-3) at 225 K, were obtained. A trace of HOCl, the reaction product from the ClONO2 + H2O --> HOCl + HNO3 reaction, was observed in addition to the Cl-2 product from reaction 1. The implications of this result for the enhancement of hydrogen chloride in the stratosphere after the El Chichon volcanic eruption and for the marine troposphere are discussed.