The electronic properties of a Ca bilayer deposited on Cu(111) have been investigated by high-resolution electron energy loss spectroscopy. For the as-deposited Ca bilayer, the surface plasmon was recorded at 3.3eV. Temperature induces a surface metal-insulator transition (evidenced by the shift of the surface plasmon energy), leading to the formation of CaO thin layers which are highly reactive towards water molecules. The presence of occupied d band in the calcium overlayer modifies surface plasmon frequency and strongly influences its dispersion relation. Results are well explained within the framework of the s-d polarization theory.