Chalcogenide glassy electrolytes have been extensively studied because of their high conductivities and their potential application as solid state electrolytes in high energy density, high performance batteries and fuel cells. The radius of the mobile cation plays an important role in determining the magnitude of the ionic conductivity and the activation energy. However, to date, very little if any research has been conducted to examine the cation radius dependence of the ionic conductivity in chalcogenide glasses. Hence, in this research the MI + M2S + (0.1Ga2S3 + 0.9GeS2) glass systems, where M = Li, Na, K and Cs, were studied in order to examine the effect of cation radius on the ion transport mechanisms in fast ionic conducting (FIC) chalcogenide glasses;The structures of the glasses in this system were studied by Raman and IR spectroscopy and it was found that the progressive formation of non-bridging sulfurs occurs with the addition of the alkali sulfides. As found in other studies of similar chalcogenide glass systems, the addition of alkali iodide causes little change in the glass structure;The ionic conductivity was found to exhibit the typical compositional dependence, increasing exponentially with alkali cation concentration. Correspondingly, the activation energy was also found to be composition dependent, showing a large difference between Li, Na, and K, Cs. The effect of the alkali radius on the activation energy was analyzed based on modified Anderson and Stuart model;For the first time, the mobile carrier density in the FIC chalcogenide glasses was determined using space charge polarization theory. The experimental impedance spectroscopy data was fitted using appropriate equivalent circuit. The binding energy was obtained through the temperature dependent of mobile carrier density. In this way, the influence of carrier density and mobility on the ionic conductivity was separated;In addition, the structures of low-alkali-content Na2S + B 2S3 (x ≤ 0.2) glasses were studied by neutron and synchrotron x-ray diffraction. Results of our diffraction and modeling studies provide direct structural evidence that doping B2S3 with Na2S creates a large number of tetrahedrally coordinated boron and S-S pairs in the glass.