We have studied electron transport in bipyridyl- dinitro oligophenylene- ethynelene dithiol ( BPDN) molecules both in an inert environment and in aqueous electrolyte under potential control, using scanning tunneling microscopy. Current- voltage ( IV) data obtained in an inert environment were similar to previously reported results showing conductance switching near 1.6 V. Similar measurements taken in electrolyte under potential control showed a linear dependence of the bias for switching on the electrochemical potential. Extrapolation of the potentials to zero switching bias coincided with the potentials of redox processes on these molecules. Thus switching is caused by a change in the oxidation state of the molecules.