The interfacial reactions between ion beam deposited diamondlike carbon (DLC) films on ZnS were studied by x-ray photoelectron spectroscopy. The DLC films were deposited by pure C+ ion bombardment. The results were also compared with those obtained from ion beam deposited DLC on Ge. It was found that the films (2-20 nm thick) deposited with a pure C+ beam in the energy range of 20-300 eV on these two substrates always showed two carbon phases with a C 1 s binding energy difference of about 0.8 eV. The low binding energy carbon phase showed an average location among about 30 samples at about, 284.4 +/- 0.2 eV whereas the high binding energy carbon phase showed it at 285.5 +/- 0.2 eV. Although carbon in a carbide form typically has C Is binding energy about 1 eV lower than that in graphite (284.7 eV), the observed difference of 0.9 eV in binding energies of the two carbon phases was not a consequence of one phase being metal carbide. Instead, the presence of these two phases appears to be characteristic of the DLC formed in this study. When the carbon in the overlayer was partly sputtered off and partly driven into the substrate by Ar+ bombardment, a carbide phase emerged with a binding energy of less than 283.8 eV for the germanium case whereas no C Is peak shift was observed in the ZnS case. Further, C/ZnS samples before and after the Ar+ bombardments showed Zn 3p and S 2p data not significantly different from those of ZnS without DLC deposition. Hence, when carbon atoms were anchored into ZnS by ion bombardment, any chemical bonds between die incorporated carbon atoms and the atoms in the substrate were much weaker than those in Ge. Adhesion tests indicated that 20 nm diamondlike films deposited on ZnS by C+ ion bombardment passed the conventional tape test and eraser-rubbing test, and those by carbon evaporation failed die tests. However, thick films deposited by ion plating adhered poorly on ZnS but very well on Ge.