The adsorption of tripiperidinophosphine oxide (TPPO) on mercury electrodes from aqueous solutions is shown to lead to formation of compact films impermeable for the Fe(EDTA) anion. Even though electrocapillary measurements indicate TPPO multilayers at the negatively charged Hg/solution interface, Fe(EDTA) kinetics are not inhibited more than would correspond to the reactant molecule being separated from Hg by a monolayer of TPPO molecules. Interestingly, the rate of electrode reactions for Fe(1,3-PDTA), Fe(CyDTA), and Co(EDTA) strongly depends on their structure. The rate constant for the one-electron reduction of the latter molecule was found to be more than three orders of magnitude lower than that for Fe(EDTA) at TPPO-covered Hg electrodes. The preexponential part of the rate law is suggested to be responsible for the observed differences in the kinetics of the electrode reactions.