In this projeot, it has been demonstrated that ouprous iodide - sulphonium iodide double salts, similar to the silver iodide systems previously studied by other workers, are sui table for use in ambient temperatuI9 batteries for low cuzrerre applications. A number of potential solid electrolytes, in which the mobile speoies is oaloium, have also been investigated, and areas for further work have been defined. The I9lavent seotion of the thesis is confidential as patent proceedings a~ possible. . The project has involved not only a search for new electrolytes, but also the development of a search strategy. In view of the limitations of di rect conductivity measurements on compaoted materials, arising from electrioal contact probl~ms, ConsidllJrable use has been made of Lndirac t methods. A critical ~view of standard electroohemical techniques, often adapted from aqueous methods, has been oarried out. This has highlighted shortcomings in the measurements of such parameters as electronic conductivi ty, where the self-discharge rate, a parameter used by battery technologists, has been found more meaningful than the results obtained fran l-/agners blocking eleotrode teohn1que. X-ray diffraction (XRD), and differential thermal analysiS (MA) have been used to disprove the commonassertion that the effect of addition of organio dopant materials to silver iodide has the effeot of stabilizing the high temperature high conductivity alpha phase. CuI based eleotrolytes have been found to differ significantly from the AgI analogues in several respects. No simple structural cri terion, such as the Group Weighting Coefficient for AgI systems, could be found to prediot the effect of addition of organic sulphonium iodide dopants. Battery cells based on CuI did not give the expected the rmodvnnmi o open circuit voltap:e (OCV), and three electrode meaauremen t a failed to elucidate this anomaly. The possible involvement of the cupric ion was explored in several ways, including the first application of Auger electron spectroscopy for this type of electrolyte system. The battery discharge characteristics showed a fairly low efficiency, but indirect evidence including the inadvertent involvement of a novel solid state aurous system, pointed to the majority of the anode material being unavailable for reaction.