Core-level photoemission spectroscopy and angle-resolved photoemission spectroscopy with synchrotron radiation have been used to study the adsorption mechanism of K on ZrC(111). The coverage-dependent measurements of the work function and the K3p core-level photoemission indicate that K adsorption proceeds via a polarization–depolarization transition. The valence-band photoemission measurements show that the Zr4d-derived surface state around the Fermi level is hybridized with the K4s orbital to form occupied bonding and unoccupied antibonding states, leading to the polarized overlayer at low coverages. The antibonding state is shifted to cross the Fermi level at the Γ̄ point at high coverages where the plasmon loss peak in the K3p core-level region is observable, which results in formation of the depolarized overlayer. The observation of the antibonding state can be viewed as direct evidence for depolarization of the overlayer.