| Summary: | The current race for space exploration has hastened the development of electrochemical technologies for the in-situ utilisation of planetary resources for the synthesis of vital chemicals such as O<sub>2</sub> and fuels. Understanding the physicochemical properties, such as the density and kinematic viscosity, of aqueous solutions is essential for the design of electrochemical devices for the electrolysis of water and CO<sub>2</sub>, particularly at low temperatures. The density and kinematic viscosity of highly concentrated Mg(ClO<sub>4</sub>)<sub>2</sub> and KOH solutions have been determined, both at low temperatures and in the presence of CO<sub>2</sub> gas. It was found that, for all of the solutions, independent of the concentration or nature of the electrolyte, as the temperature was decreased to 255 K, the density and the viscosity of the solutions increased. Upon saturation with CO<sub>2</sub>, no significant change to the density and viscosity of Mg(ClO<sub>4</sub>)<sub>2</sub>, at all of the temperatures measured, was observed. Conversely, the CO<sub>2</sub> saturated solutions of KOH showed significant changes in density and viscosity at all temperatures, likely due to the formation of carbonates. The effects of these changes on the diffusion coefficient for dissolved CO<sub>2</sub> is also discussed.
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