Dependencies of viscosity (η) and conductivity (κ) on temperature and solvent composition are examined for solutions of LiPF6 and of LiClO4 in ethylene carbonate (EC):diglyme (DG) binary mixtures to identify organic electrolyte systems with the highest conductivity as possible at room temperature. The energies of activation for viscous flow and for conductivity, determined as functions of the composition of the mixed solvent, show these two transport processes to be strongly related. Conductivities of electrolyte solutions prepared with LiPF6 and LiClO4 are comparable, but lower activation energy is required for LiClO4. A flat maximum of conductivity at about 50% (v/v) in EC is observed when the composition of the mixed solvent is varied. A high value, independent of the considered temperature, has been found for the product κ η, meaning that free ions are the predominant ionic species in these media. As a consequence, the macroscopic viscosity of electrolytes under study mainly governs their conductivity. Cyclic voltammetry, performed on negative (Li x C) and positive (Li x NiO2) intercalation electrodes in the optimized electrolyte (LiClO4 or LiPF6 (1 M)-50:50 EC:DG), gives evidence for the formation of passive layers during the first cycle and reversible electrochemical intercalation during the following cycles. DSC results show that this electrolyte system is a potential candidate for Li secondary batteries or electrochromic devices working at ambient or higher temperatures as it is safe and stable at least to 200°C.