Gaseous N2O5/air mixtures were flowed over solid NaCl at 298 K and the gaseous product ClNO2measured using FTIR. With excess NaCl, one ClNO2was produced per N2O5in the initial mixture, and from the contact time between N2O5and the salt, a lower limit to the fraction of collisions leading to reaction was estimated to be 2.5 × 10−3. This reaction is sufficiently rapid that it may lead to the formation of ppb levels of ClNO2overnight in polluted marine urban areas. The ClNO2will photolyze at dawn to give chlorine atoms which initiate the photooxidation of organics in a manner analogous to OH. This reaction may also play a role in remote Arctic chemistry if the reaction is significantly faster than our lower limit. This supports the hypothesis of Michelangeli et al. (1990) that the N2O5(g)+ NaCl(s)reaction may contribute significantly to stratospheric chemistry after the eruption of alkalic volcanoes such as El Chich