The magnetic circular dichroism (MCD) and absorption spectrum of cyclooctatetraene mononegative ion (COT-), prepared by co-condensing Cs atoms with COT neutral, have been measured in argon matrices over the temperature range 1.69-21.9 K. Attention has been focused on the first allowed electronic transition, which shows clearly resolved vibronic structure. The MCD spectrum shows a strongly temperature-dependent signal which oscillates in sign. These observations show unambiguously that COT- in its ground state is within the dynamic range of D-8h (or C-8v) symmetry and that the electronic transition involves Jahn-Teller coupling. The data can be explained if the ground and excited electronic states are assumed to have respective point group symmetries D-8h and D-4h (or D-2d) with significant Jahn-Teller effects in both states. To explain the magnitude and temperature dependence of the MCD, it is also necessary to include spin-orbit and crystal field perturbations in the ground state. The ground state can be pictured as strongly trapped in two Kekule-like structures separated by a Jahn-Teller barrier of approximate to 1440 cm(-1) with tunneling possible between the two structures. The ground state parameters are v(JT)(b(1g)) approximate to 1140 cm(-1), E(JT) approximate to 1440 cm(-1), and crystal field Delta(b(1g)) approximate to 13 cm(-1), with Ham-effect quenching of the orbital angular momentum by the factor e(-lambda 2JT) = 0.08. The excited state Jahn-Teller parameters are v(JT) approximate to 270 cm(-1), E(JT) approximate to 130 cm(-1). It is shown that the Cs+ counterion, which must be present, does not affect the analysis in any essential way.