The temperature dependences of the triplet exciton, of the vibrational modes, and of the low-energy diffraction pattern of the (111) surface of well-ordered C 60 films indicate that this surface undergoes a first-order orientational order-disorder phase transition at 225Ϯ3 K, well below the temperature of the corresponding phase transition in the bulk of the same samples, which we find to be 260 K. The bulk phase transition is therefore initiated by the surface. To our knowledge, this is the first example of a surface orientational phase transition, a phenomenon that should occur on several other molecular crystal surfaces. ͓S0163-1829͑96͒07127-5͔ Surface order-disorder phase transitions may initiate the corresponding first-order phase transitions in the solid. Surface melting has been observed to occur on surfaces of several solids 1 at temperatures substantially lower than the bulk melting point. This transition can be regarded as a complete or incomplete wetting of the ordered solid by a disordered layer.In this paper we show that also orientational orderdisorder transition in a molecular crystal may have a surface counterpart that occurs at lower temperature. This kind of surface phase transition is expected to occur on a large class of molecular crystals.Solid C 60 undergoes an orientational order-disorder transition at T B ϭ260 K and a glassy transition at about 90 K. 2-4 At temperatures above T B the quasispherical C 60 molecules rotate around their center. 5 The molecules are all equivalent and the lattice is fcc. 2 When the temperature is lowered below 260 K the rotation stops and a first-order phase transition occurs. The new lattice is simple cubic ͑sc͒ with a basis of four molecules that differ by their orientation. 3 Each molecule performs thermally activated jumps between two minima of the orientational potential. Below 90 K the jumping time becomes very long and the solid is left in an orientational glassy state. [2][3][4] The C 60 (111) surface is known to have a (2ϫ2) superstructure at TϽ230Ϯ20 K, 2,6 which results from the lowtemperature orientational ordering of the surface C 60 molecules, since the ideal (111) surface of the low-temperature sc crystal has a basis of four inequivalent C 60 molecules in a (2ϫ2) lattice. 6 Here we show that the C 60 (111) surface undergoes a first-order transition at 225Ϯ3 K, 35 K below the temperature of the bulk transition, when the surface structure changes from (2ϫ2) to (1ϫ1). This transition has been first found by measuring the temperature dependence of the surface triplet exciton spectrum and then confirmed by a jump in the scattering cross section of some vibrational modes in high-resolution electron energy-loss spectroscopy ͑HREELS͒, and by low-energy electron diffraction ͑LEED͒. The high surface sensitivity of the electron scattering mechanism that excites the triplet exciton in C 60 is proved in the Appendix.Ordered C 60 layers were grown on an Au͑110͒ substrate by sublimation from a Ta crucible in UHV (10 Ϫ10 torr͒. The C 60 powder had a purity better...