Single crystal X-ray diffraction measurements on M x Mo 3S 4 compounds ( M = Cu, Ag, Sn, In) yield evidence for the existence of a charge transfer from the interstitial element M to the Mo 6S 8 building blocks. The importance of this transfer was estimated from the contraction of the Mo 6 octahedron as a function of the valency and the concentration of the cations. It is suggested that this effect stabilises the structure and has a strong influence on the superconducting transition temperature. Bonding is discussed using a simple valence bond model which is based on the electron requirement of the Mo 6 octahedron to form 12 covalent bonds.