Infrared spectra of CHx and CDx stretches on intensively annealed, fully hydrogenated diamond crystallites are reported. By hydrogenating synthetic diamond nanocrystals at 1000 degrees C, a single sharp CHx vibrational absorption, accompanied by several weak satellites at higher frequency, has been observed at 2835.5 cm-l The bandwidth of the sharp feature is 4.5 cm(-1) and is insensitive to the temperature of measurement at 100-300 K. For CDx stretches, however, immensely broad bands centered at 2110 cm(-1) with widths of 30 cm(-1) were recorded, The dramatic difference in bandwidth reveals a vibrational relaxation lifetime of approximate to 0.2 ps for the CDx stretch on the surface. From the results of mixed isotope experiments, it is suggested that the single band at 2835.5 cm(-1) should be assigned to the CH stretch on C(111)-1 x 1, and the high-frequency satellites should be attributed to the stretches of CHx (x = 1-3) on comers, edges, or other defect sites of the crystallites. The severe band broadening in CDx is interpreted as a major result of the near-resonant coupling of the transition dipole of the adsorbate with the second-order optical phonons of the substrate, from which a relaxation time of approximate to 3 ps is predicted for CH on C(111)-1 x 1.