The interface formation between vacuum evaporated CdS and ultrahigh vacuum-cleaved CuInS₂ single crystals has been studied by synchrotron excited photoelectron spectroscopy. The valence band discontinuity is determined directly from valence band difference spectra to be DEV 50.6 (60.1) eV. This value is significantly smaller than for other preparation conditions given in the literature and evidently not suitable for solar cell applications. The similarity to observations at the CdS/CuInSe₂ interfaces suggests that neutrality levels play a dominant role in establishing the band lineup at interfaces containing chalcopyrite semiconductors.

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