Research into new energy storage technologies, for both portable and stationary applications, has become an urgent necessity. Rechargeable metal-air (oxygen) batteries are receiving intense interest as possible alternatives to lithium-ion batteries, because of their potential to provide higher gravimetric energies. While much attention has been focused on aprotic Li-O2 batteries, substantial challenges must be addressed before widespread commercialization is possible. Recently, a metal-air battery in which lithium has been replaced by sodium has received increasing attention.1 Although Na-O2 batteries (NABs) present lower gravimetric energies on a cell basis (1605 or 1108 Wh/kg based on Na2O2 or NaO2 discharge products, respectively)2 than Li-O2 batteries (3505 Wh/kgLi2O2),3 much lower charge overpotentials (~100 mV) than those in typical Li-O2 batteries (~1000 mV) have been reported based on reversible sodium superoxide (NaO2) formation. At present, Na systems have mostly been designed according to strategies developed for Li-O2 research which, though a useful initial approach, may have led to the use of suboptimal materials due to the current lack of exploration in this area. The vast majority of the work has been devoted to electrode materials, with little or negligible work on electrolytes which – as the component responsible for ion transportation between the electrodes - are key of any optimized NAB system. Glyme-based solvents have been widely used in NABs but their selection seems arbitrary and most of the studies do not consider the differences these glymes might have. In this work, we demonstrate that the selection of the solvent plays an important role on the cell chemistry and performance. The coordination number of different glymes measured by infrared spectroscopy and coupled with density functional theory revealed different solvation behavior. Moreover, the enthalpy of dissolution measured using isothermal microcalorimetry varies significantly as a function of solvent selection. In addition, the rechargeable nature of Na-O2 batteries is governed by the formation and oxidation of NaO2, making it vital to study the effect of the electrolyte in the presence of NaO2, and also to prevent the formation of irreversible side products. References 1 I. Landa-Medrano, C. Li, N. Ortiz-Vitoriano, I. Ruiz de Larramendi, J. Carrasco and T. Rojo, J. Phys. Chem. Lett., 2016, 7(7), 1161–1166. 2 P. Hartmann, C. L. Bender, M. Vračar, A. K. Dürr, A. Garsuch, J. Janek and P. Adelhelm, Nat. Mater., 2013, 12, 228–32. 3 D. G. Kwabi, N. Ortiz-Vitoriano, S. A. Freunberger, Y. Chen, N. Imanishi, P. G. Bruce and Y. Shao-Horn, MRS Bull., 2014, 39, 443–452.