During the production of molybdenum, the first reduction step of molybdenum trioxide to molybdenum dioxide is crucial in directing important product properties like particle size and oxygen content. In this study, the influence of heating rate, hydrogen flow, and potassium content on the reduction of MoO3 has been investigated via in situ X-ray powder diffraction. For low heating rates, a molybdenum bronze HxMoO3 could be confirmed as an intermediate, while γ-Mo4O11 can only be observed at high heating rates. Molybdenum formation at temperatures as low as 873 K can be controlled via hydrogen flow. The potassium content of reactants has a direct influence on the amount of Mo4O11 formed during the reaction as well as rates of Mo4O11 and MoO2 formation.