The uptake and reaction of chlorine nitrate (ClONO2) on NaCl and synthetic sea salt (SSS) powders was studied at 298 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. A time-dependent uptake coefficient was observed, with a large initial uptake coefficient measured for most samples of gamma (init) > 0.1, followed by a smaller and slowly declining uptake coefficient at longer reaction times. This behavior is shown to be consistent with uptake into, and reaction in, water on the salt surface. The steady-state uptake coefficient on NaCl was invariant over a range of ClONO2 concentrations from 10(12) to 10(13) molecules cm(-3) but was dependent on the number of salt particle layers in a manner consistent with approximately two layers of particles being available for reaction. The results of experiments using monolayers and sub-monolayers of salts, where the available reactive surface is known, give an uptake coefficient at longer times after the rapid initial uptake for ClONO2 on NaCl of gamma (t) = (6.5 +/- 3.0) x 10(-3) (2 sigma). The larger uptake coefficient obtained initially compared to longer reaction times helps to reconcile different values reported earlier by other research groups for this reaction. The uptake coefficient on SSS, which holds more water, is much larger than that for NaCl, with gamma (init) = (0.42(-0.42)(+0.46)) and gamma (t) = (0.16(-0.16)(+0.20)) (2 sigma). At the higher uptake coefficients measured for SSS, this reaction could be a significant source of Clz in the marine boundary layer if chlorine nitrate is available at a constant concentration of similar to5 ppt.