Core-level photoemission spectroscopy and angle-resolved photoemission spectroscopy with synchrotron radiation have been used to study an adsorption mechanism of K on a ZrC(111) surface. Coverage-dependent measurements of the work function and the K3p core-level photoemission reveal that the adsorbed K atoms are partially ionized in the initial stages of adsorption, and that the metallic overlayer is formed at high coverages through a continuous depolarization of the overlayer with increasing coverage. Valence-band photoemission measurements show that the Zr4d -derived surface state on ZrC(III) is hybridized with the 4s states of the adsorbed K atoms to form occupied bonding and unoccupied antibonding states at low coverages. This, together with the fact that adsorbed K is ionized in the initial stages, suggests that the bonding state should be mainly composed of the Zr4d orbitals, while the antibonding state has a dominant contribution of the K4s orbitals. As the coverage increases, the antibonding state shifts downwards and becomes an occupied state around the point in the high coverage region where the metallization of the overlayer is brought about. Since the antibonding state can be viewed as a K4s state, the observation of the antibonding state is direct evidence for the metallization of the K overlayer.