The activation enthalpy for the reduction of two pentaamminecobalt(III) complexes attached to a mercury electrode by thiophenecarboxylate ligands was evaluated from the temperature dependence of the unimolecular rate constants measured in a nonisothermal cell arrangement. The data obtained for the reactant with a bridge featuring uninterrupted conjugation support our previous opinion that the decrease of the rate constant on addition of acetone to the mixtures results from a decrease in the nuclear frequency factor owing to the overdamped solvent friction. The unimolecular frequency factors and the adiabaticity of the processes are discussed.