Molten salt electrolytes show promise as safe, effective elements of emerging low to intermediate temperature molten sodium batteries. Here we investigate the NaI-AlCl3 molten salt system for its electrochemical and physical properties at 150 and 180. C, temperatures recently used to demonstrate a new NaI battery using this molten salt system. Molten salt compositions ranging from 20-75% NaI were prepared and electrochemically interrogated with carbon fiber ultramicroelectrodes utilizing cyclic voltammetry, chronoamperometry, and differential pulse voltammetry. Results indicate that at very high or very low NaI concentrations, secondary phases present hinder diffusion of the redox-active species, potentially impacting the current density of the system. Furthermore, a concentration-independent chronoamperometric analysis technique was leveraged to determine effective diffusion coefficients of active I- in the melt phase. Collectively, the physical characterization and electrochemical properties of the tested salts indicate that the catholyte composition can significantly affect the physical state, current density, ionic diffusion, and voltage window of these promising NaI-AlCl3 molten salt battery catholytes. (c) The Author(s) 2018. Published by ECS.