Bulk (Mo2N) and supported molybdenum nitride (Mo2N/TiO2, Mo2N/CeO2) were synthesized by reduction-nitridation of bulk and supported MoO3 under a mixture of N2/H2. We investigated the mechanism of formation of molybdenum nitrides by coupling elemental analysis, in situ and ex situ XRD, Raman spectroscopy, in situ XPS/UPS spectroscopy and DFT calculation. We showed that it is possible to control the crystallographic phase and degree of nitridation (i.e. nitrogen vacancies) by adjusting the parameters of the synthesis, including the heating rate and final temperature, the nature, composition and flow of gas during reduction/nitridation and cooling. We demonstrated that β-Mo2N and γ-Mo2N go through the same intermediates: molybdenum bronze, MoO2 and Mo. The final crystallographic phase of the nitride depends mainly on the temperature rate and gas hourly space velocity employed during the synthesis, as they impact the crystallite size of the intermediates and the kinetics of reduction and nitridation. Molybdenum nitride clusters (< 0.5 nm) are formed on TiO2.