This paper describes the recovery of molybdenum from a spent CoMo/Al2O3 by soda leaching after a roasting step. First, the leaching was performed on a catalyst industrially roasted at 800 degrees C during 20 min. In order to reach a molybdenum leaching yield higher than 85%, the process was optimized using a 2(3) design of experiments with parameters such as NaOH amount, leaching time, and temperature. Unfortunately, in the best leaching conditions defined by the factorial design, the leaching yield of molybdenum never exceeded 60% because of an incomplete roasting of the catalyst proved by the X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis of the residue. In a second part, the roasting was investigated on an unroasted catalyst containing 23.9% Al, 11% Mo, 2.9% Co, 7.9% S, and 6.3% C using a central composite design. By changing the thermal treatment conditions (700 degrees C, 20 min) and performing the best soda leaching conditions (an amount of sodium hydroxide equal to the twice of the required stoichiometry and a leaching time of 4 h at 60 degrees C), it was possible to extract more than 90% of molybdenum initially present in the catalyst.