The influence of structural, thermal and electronic properties on collective electronic excitations (plasmon modes) has been investigated for thin Ag films grown at room temperature on Cu(111). For the (9 × 9)-Ag/Cu(111) structure the excitation of the two-dimensional plasmon, observed in Ag/Si(111), is forbidden by both the high corrugation of the Ag layer and the presence of the underlying metal substrate. The increase of the sp density of states at the Fermi energy induces the appearance of a broad peak a 7–8eV, assigned to a free-electron collective excitation. For increasing Ag coverage (above 20 layers), surface plasmon is confined within Ag grains, as indicated by the analysis of its dispersion relation, which is dispersionless up to a critical wave-vector. Surface plasmon confinement is removed upon annealing at 400 K. The thermal treatment induces a flattening of the Ag adlayer with a merging of Ag islands …