Active sites in Mo-based hydrotreating catalysts are produced by sulfidation. To achieve insights that may enable optimization of the catalysts, this process should be studied in situ. Herein we present a comparative XAFS study where the in situ sulfidation of Mo/δ-Al₂O₃ and Ni/δ-Al₂O₃ is compared to that of δ-Al₂O₃ supported NiMo catalysts with different NiMo ratios. The study also covers the comparison of sulfidation of Ni and Mo using different oxide supports as well as the sulfidation conditions applied in the reactor. The XAFS spectra confirms the oxide phase for all catalysts at the beginning of the sulfidation reaction and their conversion to a sulfidized phase is followed with in situ measurements. Furthermore, it is found that the monometallic catalysts are less readily sulfidized than bimetallic ones, indicating the importance of Ni-Mo interactions for catalyst activation. Mo K-edge XAFS spectra did not show any difference related to the support of the catalyst or the pressure applied during the reaction. Ni K-edge XAFS spectra, however, show a more complete sulfidation of the Ni species in the catalyst when SiO₂ is used as a support as compared to the Al₂O₃. Nevertheless, it is believed that stronger interactions with Al₂O₃ support prevent sintering of the catalyst which leads to its stabilization. The results contribute to a better understanding of how different parameters affect the formation of the active phase of the NiMo catalysts used in the production of biofuel.