The uptake kinetics of ClONO2 on NaCl (reaction 1) and on KBr (reaction 2) have been studied in a low-pressure, Teflon-coated Knudsen reactor at room temperature. The initial uptake coefficient for both reactions has been measured as 0.23 +/- 0.06 and 0.35 +/- 0.06 for reactions 1 and 2, respectively, and is independent of reactant density in the range 10(10)-10(13) molecules cm(-3). The values of the uptake coefficients are independent of presentation of the salt substrates : identical results are obtained on powder, grains, single-crystal surfaces, and thin deposited salt layers. The only product of reaction 1 is Cl-2 Reaction 2 initially produces Br-2, followed by BrCl and Cl-2. In our proposed mechanism, BrCl is the product of reaction 2, and secondary reactions between BrCl and the KBr substrate yield Br-2 at short reaction times and Cl-2 under prolonged exposure.