In-plane dispersion of the quantum-well states (QWS’s) associated with the electron confinement in metastable epitaxial Ag films grown on the Si (111) 7× 7 and Si (001) 2× 1 surfaces is investigated by angle-resolved photoemission using synchrotron radiation. In contrast to the free-electron-like behavior expected, these QWS’s show intriguing dispersions such as (i) a significant enhancement of the in-plane effective mass with decreasing binding energy and (ii) a splitting of a QWS into two electronic states with different dispersions at off-normal emission. Such unexpected electronic properties of a QWS are obviously related to the substrate band structure. Further the QWS splitting is explained by the energy-dependent phase shift of the film-substrate interface occurring at the substrate band edge.