The formation of buried K-fulleride layers by implantation of 30 keV K + into C60 films at an implant temperature of 300°C has been studied as a function of dose ranging from 1×1016 to 1×1017 cm-2. After implantation K depth profiles were measured by secondary-ion-mass spectrometry and Rutherford backscattering spectrometry. Phase characterization was done by Raman scattering and grazing-angle x-ray diffraction. Within the range of the implanted ions a transformation of the fullerene molecules to amorphous carbon (a-C) was found; however, due to the elevated implant temperature of 300°C most of the K diffuses into the depth where doping of the undestroyed film occurs. The shape of the observed K profiles depends on the dose. A local maximum of K observed right underneath the a-C layer tends to saturate at a value of about 2×1021 cm-3 for high implant doses. Both Raman scattering and x-ray diffraction strongly indicate the existence of K 3C60 regions in the implanted films. The buried K-fulleride layers are stable on air due to a passivation effect of the a-C surface layer. The formation of the K-fulleride is discussed on the basis of the phase diagram for the K-C60 system and various thermodynamic processes such as segregation, phase formation, and diffusion.