[1] Chlorate (ClO 3 À ) is an intermediate oxidation species between chloride (Cl À ) and perchlorate (ClO 4 À ), both of which were found at the landing site by the Wet Chemistry Lab (WCL). The chlorate ion is almost as stable as perchlorate, and appears to be associated with perchlorate in most terrestrial reservoirs (e.g. Atacama and Antarctica). It is possible that chlorate contributed to the ion sensor response on the WCL, yet was masked by the strong perchlorate signal. However, very little is known about chlorate salts and their effect on the stability of water. We performed evaporation rate experiments in our Mars simulation chamber, which enabled us to determine the activity of water for various concentrations. From this we constructed solubility diagrams for NaClO 3 , KClO 3 , Mg(ClO 3 ) 2 and Ca(ClO 3 ) 2 , and determined the Pitzer parameters for each salt. Chlorate salt eutectic temperatures range from 270 K (KClO 3 ) to 204 K (Mg(ClO 3 ) 2 ). Modeling the addition of chlorate to the initial WCL solutions shows that it precipitates in concentrations comparable to other common salts, such as gypsum and epsomite, and implies that chlorates may play an important role in the wet chemistry on Mars. Citation: Hanley, J