Thin MoS x (x=1.2) films are deposited on 440C steel or Si by ion beam sputtering of an MoS 2 target with (or without) high energy (160 keV) Ar + ion beam assistance. The coating microstructures of the as-deposited films are analyzed by high resolution transmission electron microscopy (HRTEM). HRTEM images of the sputter-deposited MoS x films show domains with few fringes roughly parallel to the substrate, embedded in a typical amorphous structure. This confirms the suggestion made in a previous publication that these sputtered films are amorphous with short-range order in the form of layered clusters with layers parallel to the substrate. The HRTEM images of the ion-beam assisted films show small crystallites (with no preferential orientation) within an amorphous structure. The difference between the tribological properties of these two types of coating is explained by the difference in their microstructures. HRTEM study of the wear track microstructures shows the important role played by the layered clusters which form an interfacial film accommodating sliding motion.