The amorphous compound MoS3 has been investigated by monochromatic X-ray photoemission (XPS) and vibrational spectroscopy, as well as by chemical extrusion aimed at identifying structural units inside MoS3. XPS shows that all molybdenum in MoS3 occurs in the 4+ oxidation state, while sulfur is present in at least two states, including S2- and S-2(2-) ligands in different types of coordination. This composition corresponds to a formal charge state of Mo-IV(S2-)(S-2(2-)). The emission infrared spectrum of MoS3 exhibits characteristic vibrations that are also encountered in the spectra of triangular (Mo3Sx)-S-IV cluster compounds. Three of such cluster compounds, i.e. (NH4)(2)[(Mo3S13)-S-IV]. H2O, K-5[Mo3S4(CN)(9)]. 2H(2)O, and (NEt(4))(2)[(Mo3S7Cl6)-S-IV] can be extruded from MoS3 by reactions that are essentially destructive. On the basis of these results it is proposed that MoS3 is essentially an aggregation of Mo(IV)(3)-sulfur cluster types.