The Er-Si interaction at the Er/Si(111) interface formed at room temperature is investigated by core-level and valence-band photoemission spectroscopy as well as by work-function measurements. The valence-band spectra are dominated by Er 4f emission detected in the 4\char21{}11-eV binding-energy range and by emission from Er 5d conduction-band states near the Fermi edge. The change in binding energy and shape of the Er 4f, Er 5d, and Si 2p structures is discussed in connection with the photoemission data measured on amorphous ${\mathrm{Er}}_{\mathit{x}}$${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ (0x1) silicides and on a monocrystalline ${\mathrm{ErSi}}_{1.7}$ layer epitaxially grown on Si(111). As expected, it appears that the Er 5d valence states hybridize with the Si 3sp states, while the Er 4f states, although located in the valence-band region, behave like atomiclike states. At low Er coverage [2 monolayers (ML)], the Er atoms react readily with the substrate, giving rise to Si-rich silicidelike features. It appears that a thin amorphous silicide layer grows up to about 6\char21{}8 ML. Its Er concentration is found to increase with the deposited Er thickness, as evidenced by the valence band and Si 2p core-level evolution from Si-rich to Er-rich silicide signatures. We also find that the total amount of reacted Si remains quite constant for Er coverages above 3 ML. Furthermore, the valence-band data clearly reveal a heterogeneous growth involving a mixture of pure Er clusters and silicide patches at coverages as low as 3 ML. The reaction stops above 6\char21{}8 ML with the formation of a pure Er-metal overlayer.