The adsorption and the corrosion inhibiting effect of benzotriazole (BTAH) on Cu(100) electrodes in 0.1 M HCl were investigated using in situ STM, in situ FTIR spectroscopy, and electrochemical measurements. In the double-layer range up to potentials of -0.6 V vs SCE a Cu surface morphology with extended, atomically flat terraces, separated by almost randomly oriented steps, and a low Cu surface mobility are observed by STM. High-resolution STM images reveal a commensurate superstructure in this potential range, which is attributed to a chemisorbed adlayer of BTAH molecules. At potentials around -0.6 V this structure is replaced by a c(2 x 2) Cl- adlayer, which has the same atomic and long-range structure as found in BTAH-free HCl solution. Upon further potential increase to potentials >-0.35 V STM experiments and polarization measurements indicate the onset of Cu dissolution, while the surface is still covered by the c(2 x 2) Cl- adlayer. At slightly higher potentials (>-0.3 V) STM, IR, and electrochemical data point to the formation of a thick, inhibiting Cu(I)BTA film on the Cu surface. The pronounced differences of the BTAH adsorption behavior to that found in H2SO4 solution provide a microscopic explanation for the reduced inhibition efficiency of BTAH in the presence of chloride.