We present high-resolution electron energy loss spectroscopy (HREELS) measurements on surface plasmon (SP) dispersion in systems exhibiting quantum well states (QWS), i.e. Na/Cu(111), Ag/Cu(111), and Ag/Ni(111). Our results demonstrate that the dominant coefficient of SP dispersion for thin and layer-by-layer Ag films presenting QWS is quadratic even at small q||, in contrast with previous measurements on Ag semi-infinite media and Ag thin films deposited on Si(111). We suggest that this behavior is due to screening effects enhanced by the presence of QWS shifting the position of the centroid of the induced charge less inside the geometrical surface compared with Ag surfaces and Ag/Si(111). For ultrathin Ag films, i.e. two layers, the dispersion was found to be not positive, as theoretically predicted. Annealing of the Ag film caused an enhancement of the free-electron character of the QWS, thus inducing a …