Atomic-resolution in situ scanning tunneling microscopy (STM) observations of (100) Cu electrodes in H2SO4 solution containing 10(-6) to 10(-4) M benzotriazole (BTA) are presented. At potentials less than or equal to -0.6 V vs. a Hg/Hg2SO4 (K2SO4 sat.) reference electrode ordered super structures of a chemisorbed BTA adlayer as well as a disordered, chainlike structure, suggesting a polymerized BTA film, were observed. Cu corrosion commences at potentials less than or equal to -0.4 V by the formation of monoatomic etch pits and slow step flow etching, resulting in roughening of the Cu surface. On changing the potential back to less than or equal to -0.6 V the surface becomes level again due to redeposition of dissolved Cu at the Cu steps.