We report the results of our investigation on the electron spin relaxation mechanism of the monoanion of C-60 fullerene in liquid solution. The solvent chosen was carbon disulfide, which is rather uncommon in EPR spectroscopy but proved very useful here because of its liquid state over a wide temperature range. The conditions for exclusive formation of the monoanion of C-60 in CS2 were first determined using electrochemical measurements. Using these results, only the monoanion of C-60 was prepared by chemical reduction using Hg2I2/Hg as the reducing agent. The EPR line width was measured over a wide temperature range of 120290 K. The line widths show weak dependence on temperature, changing by a factor of only about 2, over this temperature range. We show that the observed temperature dependence does not obey the Kivelson-Orbach mechanism of electron spin relaxation in liquids, applicable for radicals with low-lying, thermally accessible excited electronic states. The observed temperature dependence can be empirically fitted to an Arrhenius type of exponential function, from which an activation energy of 74 +/- 3 cm(-1) is obtained. From the qualitative similarities in the characteristics of the spin relaxation rates of C-60 monoanion radical and the cyclohexane type of cation radicals reported in the literature, we propose that a pseudorotation-induced electron spin relaxation process could be operating in the C-60 monoanion radical in liquid solution. The low activation energy of 74 cm(-1) observed here is consistent with the pseudorotation barrier of C-60 monoanion, estimated from reported Jahn-Teller energy levels.