[TCNE].− (TCNE=tetracyanoethylene) has been isolated as D2h π–[TCNE]2 2− possessing a long, 2.9 Å multicenter 2-electron-4-center (2e−/4c) C−C bond, and as C2 π–[TCNE]2 2− possessing a longer, 3.04 Å multicenter 2e−/6c (4 C+2 N atoms) bond. Temperature-dependent UV/Vis spectroscopic measurements in 2-methyltetrahydrofuran (MeTHF) has led to the determination of the dimerization, 2[TCNE].−⇌π–[TCNE]2 2−, equilibrium constants, Keq(T), [[TCNE]2 2−]/[[TCNE].−]2, enthalpy, ΔH, and entropy, ΔS, of dimerization for [Mepy]2[TCNE]2 (Mepy=N-methylpyridinium, H3CNC5H5 +) possessing D2h π–[TCNE]2 2− and [NMe4]2[TCNE]2 possessing C2 π–[TCNE]2 2− conformations in the solid state; however, both form D2h π–[TCNE]2 2− in MeTHF solution. Based on ΔH=−3.6±0.1 kcal mol−1 (−15.2 kJ mol−1), and ΔS=−11±1 eu (−47 J mol−1 K−1) and ΔH=−2.4±0.2 kcal mol−1 (−10.2 kJ mol−1), and ΔS=−8±1 eu (−32 J mol−1 K−1) in MeTHF for [NMe4]2[TCNE]2 and [Mepy]2[TCNE]2, respectively, the calculated Keq(298 K) are 1.6 and 1.3 m−1, respectively. The observed Keq (145 K) are 3 and 2 orders of magnitude greater for [NMe4]2[TCNE]2 and [Mepy]2[TCNE]2, respectively. The Keq(130 K) is 4470, 257, ≈0.8, and ≪0.1 m−1 for [NMe4]2[TCNE]2, [Mepy]2[TCNE]2, [NEt4]2[TCNE]2, and [N(nBu)4]2[TCNE]2, respectively, decreasing with increasing cation size. At standard conditions and below ambient temperature the equilibrium favors the dimer for the NMe4 + and Mepy+ cations. From the decreasing enthalpy, NMe4 +>Mepy+, along with the decrease in dimer formation Keq(T) as NMe4 +>Mepy+>NEt4 +>N(nBu)4 +, the dimer bond energy decreases with increasing cation size in MeTHF. This is attributed to a decrease in the [A]+⋅⋅⋅[TCNE]− attractive interactions with increasing cation size. Solid state UV/Vis spectroscopic determinations of [NMe4]2[TCNE]2 are reported and compared to D2h π–[TCNE]2 2− conformers. The feasibility and limitations of temperature-dependent electron paramagnetic resonance (EPR) measurements for the determination of Keq(T) are also discussed.