High-pressure-temperature (P-T) experiments were conducted in an attempt to determine the diffusion rates of C atoms in diamond, and the possibility of changes in the isotope compositions of diamond at high P-T in the Earth's mantle. The starting material consisted of a polished plate of natural diamond (very largely 12C), which had been coated with 13C diamond by chemical-vapour deposition to form a sharp interface between 12C and 13C diamond. Three experiments were performed at 1800, 2000 and 2300°C, all at 7.7 GPa, for 0.5-20 h. Isotopic profiles obtained by ion microprobe before and after each experiment showed no evidence of relaxation of the sharp interface between 12C and 13C, and so diffusion must have been on a scale less than the ∼32 nm depth resolution for this technique. Using 32 nm as the maximum length scale of diffusion across the interface, the maximum ln D (diffusion coefficient) values for the experiments were calculated to be in the range -38 to -42. Unlike previous experimental data, these results show that changes in the isotopic compositions of diamond on long time scales in the Earth's upper mantle are unlikely. Furthermore, the results support empirical evidence from mapping of C isotope distributions in natural diamonds that C isotope compositions reflect diamond growth compositions.