C60 films have been implanted with 30 keV K+ ions with doses from 1×1014 to 1×1016 cm-2 at target temperatures up to 350°C. Conductivity measurements performed in situ show a decrease in sheet resistivity with increasing dose for fullerene samples implanted at elevated temperatures, whereas implantation at room temperature results in no change of resistivity. After exposing the samples to air the resistivity remains constant over weeks. Raman scattering showed that an amorphous surface layer is formed by the implantation process and that the fullerene molecules beyond this layer remain undestroyed. Secondary ion mass spectrometry and Rutherford backscattering indicate that at elevated implant temperatures K diffuses into deeper regions of the film whereas oxygen is present only in a surface layer about 10 nm thick.