Bilayers consisting of a self-assembled alkanethiol monolayer of variable chain length and Langmuir-Blodgett monolayers of 16-ferrocenylhexadecanoic acid were fabricated on Au electrodes. The hydrophobic ferrocene groups reside at the monolayer-monolayer interface and are variably spaced from the underlying electrode. Interfacial electron transfer rates k(o) for ferrocene electron transfer across the alkanethiol spacer monolayer were measured cyclic voltammetrically. The rates were found to fall off exponentially with increasing alkanethiol chain length, giving a decay constant of -0.96 per methylene. The electron transfer rate with a dodecanethiol spacer layer (k(o) = 12 s(-1)), which involves a non-bonded donor-acceptor couple, is almost 2 orders of magnitude lower than those for the analogous ferrocenes chemically bound on Au through alkane thiols of similar bond length. The higher rate in the latter may be attributed to a through-bond mechanism, as opposed to the former in which there exists a single "break" in the chemical bond network. The results have their implications on the electron transfer studies of biological systems.