Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCl n ; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (M n + ) on redox reactions accessible to lanthanide ions; Ln 3+ + e 1− → Ln 2+ (Ln = Eu, Yb, Sm; e 1− = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu 2+ instantaneously formed when Eu 3+ dissolved in molten chloride salts that had strongly polarizing cations (like Ca 2+ from CaCl 2 ) via the Eu 3+ + Cl 1− → Eu 2+ + ½Cl 2 reaction. Conversely, molten salts with less polarizing outer-sphere M 1+ cations ( e.g. , K 1+ in KCl) stabilized Ln 3+ . For instance, the Eu 3+ /Eu 2+ reduction potential was >0.5 V more positive in CaCl 2 than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard M n + cations (high polarization power) inductively removed electron density from Ln n + across Ln-Cl M n + networks and stabilized electron-rich and low oxidation state Ln 2+ ions. Conversely, less polarizing M n + cations (like K 1+ ) left electron density on Ln n + and stabilized electron-deficient and high-oxidation state Ln 3+ ions. Molten salt matrices were used to evaluate outer-coordination sphere effects on lanthanide redox chemistry. Results were rationalized by correlating the polarization power of the outer-sphere cation with shifts in the Ln 3+ /Ln 2+ reduction potentials.