The sulfidation of uncalcined and calcined alumina-supported cobalt and cobalt-molybdenum catalysts was systematically studied by means of in situ Mössbauer emission spectroscopy (MES) at room temperature. The spectra obtained during the stepwise sulfidation of the uncalcined catalysts clearly resemble those observed for carbon-supported ones. Hence, the interpretation of the spectra of the alumina-supported catalysts is based on the conclusions drawn from the MES studies of the carbon-supported catalysts, which are less complex because Co ions do not diffuse into the support. It is demonstrated that not only in sulfided CoMo/Al2O3, but also in sulfided Co/Al2O3, catalysts Co-sulfide species with a "Co-Mo-S"-type quadrupole splitting can be formed. It is concluded that the Co-sulfide species formed in sulfided Co/Al2O3 and CoMo/Al2O3 catalysts are essentially the same, only the particle size and ordering of the Co-sulfide species may differ, as in the case of Co/C and CoMo/C catalysts. The function of the Mo, which is present as MoS2, is merely to stabilize very small Co-sulfide particles, which in the limit contain only one single Co atom. Furthermore, it turns out that the value of the electric quadrupole splitting (Q.S. value) of the Co-sulfide phase in the sulfided catalysts depends on the sulfiding temperature and Co content. This observation leads to the conclusion that large Q.S. values point to the presence of very small Co-sulfide entities or particles (the lower limit being "particles" containing only one Co atom, such as proposed in the "Co-Mo-S" model), whereas small Q.S. values point to the presence of large Co-sulfide particles (the upper limit being crystalline Co9S8).