Horse heart cytochrome c was immobilized on mixed self-assembled monolayers (SAMs) of carboxyl-and methyl-terminated alkanethiolates (HOOC(CH2)(10)S + CH3(CH2)(9)S) on Au(111) electrodes. The apparent standard rate constant k(obs) of electron transfer for the electrode system of cytochrome c\mixed SAM \gold electrode was measured by a.c. impedance and a.c. modulated UV-visible electroreflectance techniques. A strong dependence of k(obs) on the composition of the mixed alkanethiol solution, from which the monolayers had been assembled, was observed. A maximum of k(obs) approximate to 200 s(-1) was found for SAMs formed from solutions containing a mole fraction chi approximate to 0.8 of carboxyl-terminated alkanethiols. This rate constant is about six times faster than that of a system with a single-component monolayer of HOOC(CH2)(10)S (chi = 1). Reductive desorption suggested that the two alkane thiolates mix homogeneously in the monolayer and there is no sizable phase separation on the Au(111) electrode. Double-layer capacitance measurements determined the degree of protonation of the mixed SAMs in the pH range 6-9. The enhancement of electron-transfer rate is attributed to a favorable distribution of protein binding sites in the mixed monolayers.