The surface structures of C60films, epitaxially grown on the GeS(001) surface, were investigated from 90 to 350 K by helium atom scattering (HAS) diffraction. The present HAS results indicate a step-flow growth mode that is consistent with the results of previous x-ray scattering studies. By monitoring the diffraction intensities, the orientational-disordering phase transition is found to be completed at Tc=235K, which is about 25 K lower than the bulk transition temperature. This surface phase transition appears to be preempted by rotational disordering of C60molecules at defect sites, already initiated at Ts=130K. ©1997 American Institute of Physics.