The development of ecofriendly electrolytes for lithium-ion batteries is one of the actual tasks of modern electrochemistry. In particular, to this purpose, the highly concentrated ternary aqueous systems based on lithium acetate (LiOAc) have been actively investigated. Here, the diffusion coefficients of 7Li+ and 133Cs+ cations, OAc– anion, as well as water (1H), in ternary aqueous solutions of cesium and lithium acetates in a range of temperature (– 15 ÷ 35 °C) have been measured using the PFG NMR method. A direct attempt to interpret the obtained dependences within the framework of the Stokes–Einstein model led to the fact that the calculated hydrodynamic radius of the Cs+ cation turned out to be noticeably smaller than its crystallographic one. An approach to describing the high rate of diffusion of cesium cations is proposed, based on taking into account the local viscosity near cations of both types. The use of the approach allowed us to calculate more correctly the hydrodynamic radii of cations, while remaining within the framework of the Stokes–Einstein model. As a result, it has been possible to describe the features of translational motion of components in a complex system that is interesting for electrochemical applications.