Across the southern highlands of Mars, approximately 640 sites of chloride-bearing deposits have been identified using the Thermal Emission Imaging System (THEMIS). These isolated, irregularly shaped patches (approximate size range of 0.33 – 1300 km2, with an average size of 24 km2) have been dated to the older geologic periods on Mars: Noachian (4.5 – 3.5 billion years ago) and Hesperian (3.5 – 2.9 billion years ago) periods. On Earth, chlorides are known to form through aqueous processes. Similar processes are expected to be responsible for the formation of chloride deposits on Mars. The finding of these deposits is significant in that it provides further evidence for the presence of surface or subsurface water in ancient Mars.