Homoepitaxial diamond films grown by chemical vapor deposition from12C and13C containing gases have been studied by Fourier transform infrared spectroscopy. A sharp absorption band observed at 3123 cm−1is attributed to a CH vibrational absorption. It resembles the previously observed 3107 cm−1absorption band in natural diamond. Isotopic replacement of carbon, hydrogen, and nitrogen by13C,2H, and15N reveals, that a CH center without nitrogen participation is responsible for this new absorption. Furthermore, new hydrogen related electronic transitions have been observed around 7300 cm−1. These lines shift to higher energies in13C crystals, in accordance with the increase of the electronic band gap. ©1995 American Institute of Physics.