Kinetic parameters for heterogeneous electron transfer are reported for decamethylferrocene in two low-temperature solvent systems : butyronitrile and 2:1 (v:v) ethyl chloride/butyronitrile, The(Me(5)C(5))(2)Fe-+/0 rate constant ranges from 4 x 10(-6) to 10(-3) cm/s over the temperatures investigated. Temperature-dependent heterogeneous electron-transfer rate constants used for the determination of activation parameters were obtained from comparison of experimental cyclic voltammograms to those produced via digital simulation employing Marcus-DOS kinetic theory. The activation parameters are similar in the two solvents (Delta G* = 0.27 and 0.26 eV) and are somewhat larger than predicted for an outer-sphere reorganizational barrier from dielectric continuum theory (Delta G* = 0.22 and 0.21 eV). Available literature values for the inner sphere barrier for (Me(5)C(5))(2)Fe-+/0 and the thermal activation of solvent dipole longitudinal relaxation times are compatible with the experimental barriers containing small contributions from these sources. This work represents the lowest temperature electrochemical kinetic study to date employing a diffusing redox species in liquid solution.