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Disproportionation of Cu(I)X is the major step in Single‐Electron Transfer Living Radical Polymerization (SET‐LRP). The disproportionation of Cu(I)X mediated by Me₆‐TREN in various solvents was studied through UV–vis spectroscopy and Dynamic Light Scattering (DLS). UV–vis experiments reveal that disproportionation is dependent on both solvent composition and concentration of Me₆‐TREN, consistent with a revised equilibrium expression and corroborated by mathematical models. Electrochemistry data do not accurately predict the extent of disproportionation in the presence of Me₆‐TREN. Exemplified by DMSO, a favored solvent for SET‐LRP, UV–vis spectroscopy shows that under certain conditions disproportionation is four‐orders of magnitude greater than the value reported from electrochemistry experiments. Through UV–vis and DLS analysis, it was demonstrated that DMSO, DMF, DMAC, and NMP …