Electron microscopy, X-ray diffraction and Auger spectroscopy studies were made on unsupported Co–Mo sulphides in order to elucidate the influence of preparation on the structural properties of these solids. The effect of varying the cobalt loading in two series of catalysts prepared by different methods was studied. The precipitation (HSP) and the impregnation (ITD) methods were used. X-ray and electron diffraction patterns showed that the crystallinity of Co9S8 in mixed sulphides depends strongly on the method of preparation. Scanning micrographs revealed differences in morphology for both methods, precipitated samples show irregular highly porous particles, whereas impregnated samples show well defined shapes for MoS2 particles and over them some aggregates as a function of the preparation method. The present results show that the final form of cobalt in mixed sulphides depends strongly on the way of preparation. The impregnation procedure allowed a good distribution of cobalt on the MoS2 surface mainly by forming a poorly crystalline form of Co9S8 and atomically dispersed cobalt. © 1989, Elsevier Science & Technology. All rights reserved.