X-ray powder diffraction with synchrotron radiation was used for the first time to study in situ diamond crystallization from the Fe-Ni-C melt at pressures up to 6 GPa and temperatures up to 1700 K. At 5.2 GPa over the whole temperature range of diamond crystallization (1510-1605 K), the melt is in equilibrium with both diamond and fcc Fe-Ni-C solid solution (gamma-phase); that is, the L = C + gamma monovariant eutectic reaction takes place. From the non-isothermal kinetic data, it follows that diamond crystallization is controlled by carbon diffusion in the melt. Kinetic data are best fitted by the model that assumes a constant nucleation rate and a three-dimensional growth of nuclei. with an apparent activation energy of 148(64) kJ mol(-1).