An equilibrium isotope effect (confirmed via physical separation of the isotopic isomers involved) was observed via the EPR analysis of a mixture of benzoquinone and benzoquinone-C-13(6) competing for a deficient number of electrons in a tetrahydrofuran-liquid ammonia mixture. The K(eq) for the reaction C6H4O2- + 13C6H4O2 = C6H4O2+ 13C6H4O2- is 0.66 +/- 0.04 at 200 K. An analogous competition for electrons between benzoquinone and perdeuterated benzoquinone shows no equilibrium isotope effect. The C-13 effect is attributed almost exclusively to the carbonyl carbons. The low spin density on the non-carbonyl positions renders the substitution of the protons with deuteriums and substitution of the non-carbonyl carbons with C-13 ineffective in altering the solution electron affinity.