Plasmon dispersion in epitaxial graphene systems has been studied by angle-resolved electron-energy-loss spectroscopy. In quasi-freestanding graphene/Pt(111), characterized by the survival of the Dirac cone of pristine graphene, the screening by the underlying substrate changes the dispersion of the intraband plasmon from square-root-like to an acoustic-like behavior. Likewise, the interband plasmon assumes a quadratic dispersion instead of the linear dispersion expected for free-standing graphene. By contrast, two-dimensional plasmon modes in graphene/Ni(111) are supported by interface states due to the strong hybridization between Ni d bands and π states of graphene. Both the intraband and the interband plasmon in graphene/Ni(111) show a square-root-like behavior dispersion, typical of a two-dimensional electron gas (2DEG). The intercalation of Cs atoms restores the linear plasmon dispersion of …