A variable-temperature scanning tunneling microscope (STM) was used to perform single-molecule chemistry and vibrational spectroscopy on benzene (C6H6), pyridine (C5H5N), and their isotopes adsorbed on Cu(001) at 8 K. Inelastic electron tunneling spectra taken with the STM (STM-IETS) showed a C-H stretch mode for pyridine but not for benzene. Individual pyridine and benzene molecules were dissociated by tunneling electrons. The fragments of each parent molecule were imaged and the vibrational spectra recorded. A C-H stretch mode was detected by STM-IETS for the fragments of both molecules. The detection of a C-H stretch mode in the benzene fragment is attributed to a change in bonding geometry with respect to the parent. The images and spectroscopic data are consistent with the production of C5H4N and C6H4 in the dissociation of pyridine and benzene, respectively.