The relationship between the activities and surface properties of the carburized 1.0-18.7 wt% Mo/Al₂O₃ by X-ray photoelectron spectroscopy (XPS) was studied in the hydrodesulfurization (HDS) of thiophene using a microreactor at 573 K and total pressure of 0.1 MPa. The carburization of 1.0–18.7 wt% Mo/Al₂O₃ catalysts were carburized by a temperature-programmed reaction from 573 to 973 K in a stream of 20% CH₄/H₂, and then in situ supplied to the activity measurement. The carburized 11.6 wt% Mo/Al₂O₃ catalyst was 3 times more active than the sulfided catalyst and as active as the sulfided commercial CoMo/Al₂O₃ for an 8 h run time. Carburization of the Mo/Al₂O₃ catalyst prevented the rapid deactivation. Based on the XPS analysis, the Mo atom in the carburized 11.6 wt% Mo/Al₂O₃ catalyst was sulfided to MoS_0.34 at 0.167 h (MoS_0.39 at 8 h). Consequently, sulfur was deposited on Mo carbide and changed to Mo sulfide which was less active than the original Mo carbide. From the deconvolution of the Mo 3d envelope, the HDS activities of the Mo carbide catalysts were linearly related to the presence of the Mo⁴⁺ ions of the Mo carbide catalyst.