Epitaxial layers of ZnS were grown on cleaved GaP(110) surfaces by molecular beam epitaxy in an ultrahigh vacuum photoelectron spectrometer. The growth mode and the structure of the overlayer were studied by means of low-energy electron diffraction (LEED) and core as well as valence level photoemission using synchrotron radiation. The attenuation of substrate core-level intensities with ZnS deposition indicate layerwise growth. LEED demonstrates the growth of the cubic (zinc-blende) phase as expected for substrate-stabilized growth. A minor interface reaction is evident from changes in the appearance of the substrate (Ga 3d) and overlayer (S 2p) core levels with increasing thickness. S-Ga bonding was observed in a thin interfacial layer. The valence band offset for this lattice-matched heterojunction interface system was determined, and found to be of the straddling type (type I); its magnitude is in agreement with predictions based on the dielectric midgap energy model.