Menadione (2-methyl-1,4-naphthoquinone) reduction and menadiol oxidation at octadecanethiol (C18SH) monolayer modified gold electrodes were investigated by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). The modified electrode acts as a better barrier toward the ferrocyanide transport than toward the menadione species. This difference is attributed to permeation of the organic substrates into the hydrophobic monolayer. A simple model is proposed and applied to extract the rate constant of the kinetically limiting permeation step from the cyclic voltammograms, However, a better estimate of the transport properties is obtained by SECM. The same trends are observed with CV and SECM, and a similar pH dependence shows the loss of an intermediate formed during menadione reduction from the monolayer with increasing pH. This loss can probably be assigned to the rapid expulsion of the more hydrophilic reduced species from the monolayer.