The relative efficiencies of electron tunneling across self-assembled monolayers (SAMs) of n-alkanethiols and n-alkaneselenols, CH3-(CH2)(n)-XH, where n = 8, 9, 11, and X = S or Se, deposited on mercury electrodes were measured via electroreduction of Ru(NH3)(6)(3+) in aqueous solutions. Electron tunneling rates across the monolayer films decay exponentially with the monolayer thickness with a tunneling coefficient, beta = 1.1 +/- 0.1 per CH2 irrespective of the identity of the -XH headgroup. Electron tunneling rates across n-alkanethiol monolayers are ca. 4-fold larger than the rates measured across n-alkaneselenol monolayers containing the same number of carbon atoms, signifying the importance of headgroup/metal contact resistance in electron transfer through SAMs oil mercury.