[en] Sharp absorption lines produced by hydrogen in diamond have been studied in diamonds grown from ¹³C by high-pressure, high-temperature synthesis. Analyses of the two-phonon intrinsic absorption and of the Raman shift have shown that the ¹²C content of the diamond is 13.5%. This value is entirely consistent with the relative intensities of the absorption lines at 3107 and 3098 cm^-1, confirming that they are due to a strongly localized C-H stretching vibration. A correlated line, observed at 1405 cm^-1 for natural diamond, and attributed to the C-H bending vibration, exhibits both an isotope shift and a shift due to weak coupling with the one-phonon modes. By analogy with natural diamond, lines for the ¹³C diamond at 2757 and 4469 cm^-1 are assigned to the combinations 2 h-bar ω_b and h-bar ω_s + h-bar ω_b, respectively, where h-bar ω_s and h-bar ω_b are the stretch and bend frequencies. The frequency invariance, on carbon isotope substitution, of a hydrogen-related line at 3237 cm^-1 is not inconsistent with the proposal that this is produced by a N-H vibration