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Controlled step-by-step dissociation of single iodobenzene molecules has been performed at Cu(111) step-edges using tunneling electrons from a scanning tunneling microscope (STM) tip at 12 K. We show that the threshold tunneling electron energies to break a single bond inside a polyatomic molecule can be determined by using I–V spectroscopy of single molecules. Electron energies of 1.5±0.1 eV are necessary to break the σ_C–I bond. This selective bond breaking procedure is possible due to the inherent bond energy differences inside the molecule. It requires electron energies of 3.1±0.1 eV or higher to further disrupt the π ring.