The temperature-programmed reduction of molybdenum trioxide (MoO3) from 300 to 773 K in 50 vol% hydrogen in He (10(5) Pa) at different heating rates (0.1. 0.2, and 5 K/min) was investigated by in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). At heating rates of similar to0.1 and similar to0.2 K/min, the formation of the molybdenum bronze H0.34MoO3 was observed by in situ XRD in the early stage of the reduction of MoO3. At a heating rate of 5 K/min, the formation of a more disordered bronze (HxMoO3 with x similar to0.07) prior to the detection of the product phase MoO2 was observed by in situ XAFS. In both studies, the consumption of the bronze was found prior to the complete reduction of MoO3. A simplified mechanism for the temperature-programmed reduction of MoO3 in hydrogen is proposed that includes: (i) incorporation of H-2 in the MoO3 bulk and formation of a more or less ordered bronze, (ii) consumption of the bronze and formation of nucleation site for MoO2. (iii) nucleation of MoO2 and nuclei growth.