Molybdenum-based catalysts were prepared by impregnation of gamma-alumina with heteropolycompound (HPC) solutions: two salts were used, a nonreduced compound (Co3/2PMo12O40) and a reduced one (Co7/2PMo12O40) The dried and calcined solids were characterised with laser raman and diffuse reflectance spectroscopies. Extraction of the surface entities of the oxidic precursors was performed by washing them with various electrolytic solutions and the filtrates analysed by polarography. On the basis of these characterisations, a detailed description of the evolution of the entities during the preparation as well as the nature of the surface oxomolybdate species of the oxidic precursor have been deduced. It has been shown that the nonreduced HPCs were decomposed upon impregnation, whereas the reduced HPCs were preserved on the support after impregnation and drying. After calcination for 4 h at 673 K tin air for the nonreduced and in air or N-2 for the reduced compounds), the HPCs were decomposed. However, it appears that the nature of the surface oxomolybdenum phase of the calcined solids strongly depends on the nature of the starting material. This difference should explain the variation in the thiophene conversion.