Electron-transfer rates in ferrocene-containing alkanethiolate monolayers on gold electrodes were measured as a function of temperature using fast-scan cyclic voltammetry. Reorganization energies for the ferrocene oxidation half-cell reaction obtained from fits to peak potential versus log (scan rate) data are compared with activation energies obtained from Arrhenius analyses of the standard rate constants measured at different temperatures. The Arrhenius analysis yielded activation energies that were consistently lower than one-fourth of the reorganization energy obtained from analyses of individual fast-scan voltammetry data sets at a fixed temperature. This discrepancy is interpreted in terms of a non-zero entropic contribution to the activation energy. The magnitude of this entropic contribution is consistent with expectations based on the reported reaction entropy for the ferrocene half-cell reaction in water.