We have used scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) to elucidate the structure and thermodynamics of the (2√3 × 2√3)R30° phase of C 60 on Ag(111), which consists of a mixture of molecules in two different site/orientation states. The structure analysis identifies the two types of molecules as (1) sitting on a vacancy with a hexagon face down and (2) sitting on a top site with a C-C bond down. The molecules flip between the two states at a temperature-dependent rate. We show using a thermodynamic analysis that the two states differ by 0.07 eV and are separated by an energy barrier of 0.84 eV. Their dynamical equilibrium involves the diffusion of surface vacancies between C 60 molecules, producing spatially and temporally correlated flipping events. © 2012 American Physical Society.