Laboratory measurements of heterogeneous reactions of Cl2(g) on solid sea salts at ambient temperature and low relative humidity have been carried out using a Knudsen cell reactor. Synthetic sea salt and commercial natural sea salt are used to represent sea salt particles. The uptake probabilities for synthetic sea salt (&ggr;=(2.2¿0.3)¿10-2) and for natural salt (&ggr;=(3.1¿1.1)¿10-2) have been obtained and Br2 is observed as the predominant product. The heterogeneous reactions of Cl2 with solid pure NaBr and KBr: 1/2Cl2(g)+MBr(s)→MCl(s)+1/2Br2(g) (M=Na, K), have been investigated. These results show that synthetic and natural salts have a similar extent of reactivity compared with solid pure bromides in spite of their small Br fraction. These reactions are expected to be an important Cl2 sink and Br2 source in the marine boundary layer. ¿ 1998 American Geophysical Union