One method frequently employed for the preparation of active oxide catalysts consists of partial reduction under hydrogen at elevated temperatures. In this process, it is important to identify well-defined suboxides that can have high catalytic activity and are stable at the elevated temperatures typical of many catalytic reactions. Our results for the reaction of H-2 with alpha-NiMoO4 and beta-CoMoO4 show that in situ time-resolved X-ray diffraction is a powerful technique to study the reduction/activation of mixed-metal oxides. It is clearly shown that the mechanism for the reduction of a mixed-metal oxide catalyst can exhibit drastic changes with respect to that observed for simple metal oxide catalysts. The generation of stable suboxides is difficult to predict. Thus, the reaction of H-2 with alpha-NiMoO4 does not lead to formation of a well-ordered NiMoOx intermediate. On the other hand, during the reduction of beta-CoMoO4, Co2Mo3O8 and/or CoMoO3 are formed. These chemical transformations are accompanied by changes in the line shape and position of the MoLII-edge in XANES and affect the behavior of reduced NiMoO4 and CoMoO4 catalysts. Induction times were detected in the reduction process of CoMoO4. From the present results and data previously reported for NiO, it is clear that this phenomenon should be taken into consideration when aiming at the activation of oxide catalysts via reduction in H-2.