Structural and electronic properties of the clean InP(001) surface and the CdS/InP(001) interface were investigated by soft x-ray photoemission and reflectance anisotropy spectroscopy (RAS). Clean InP(001) (2 x 4) surfaces were prepared by desorption of a protective arsenic/phosphorus double capping layer, thereafter, CdS deposited by single source molecular beam epitaxy at a substrate temperature of 200 degrees C. Core-level and valence-band photoemission spectra, as well as the RAS data, were taken ia situ. Core-level spectra show a disappearance of In 4d and P 2p surface core-level shifts and a chemical reaction between indium and sulphur upon CdS deposition. From valence-band spectra, a total valence-band offset of Delta E-v = (0.75 +/- 0.10) eV is derived, which corresponds to previous results of the CdS/InP(110) interface. After deposition of thin CdS layers, RAS spectra show strong features close to interband critical points of bulk InP due to surface roughening induced by the interface reaction. A feature around 5.3 eV, developing with higher coverage, indicates the formation of the metastable cubic phase of CdS as confirmed by additional Raman spectra.