The progress of electron exchange between 4-hydroxy-2,2,6,6-tetramethyl-piperidinyl-1-oxy (TEMPOL) and a hanging mercury drop electrode can be controlled by changing the oxidation state of monolayer of 1,1'-diheptyl-4,4'-bipyridinium dication (heptyl viologen) adsorbed on the electrode; the monolayer is ''open'' in the oxidized state and ''closed'' in the reduced state.