Horse heart cytochrome c was immobilized electrostatically on self-assembled monolayers (SAMs) of carboxylic-acid-terminated alkanethiols, HS(CH2)(n)COOH (n = 2, 10) on gold electrodes. The rate constants of the electrode reaction of cytochrome c at the SAM-modified gold electrodes were determined by the frequency dependence of a.c. modulated UV-visible electroreflectance signals. The apparent standard electrode reaction rate constants of cytochrome c were 880 s(-1) and 72 s(-1) at the HS(CH2)(2)COOH and HS(CH2)(10)COOH SAM modified gold electrodes respectively. The rate constant at HS(CH2)(2)COOH was much smaller than expected from Marcus theory. However, the rate constant at HS(CH2)(10)COOH could be explained in terms of the through-bond tunnelling mechanism with tunnelling parameter beta = 8.2 nm(-1).