Inorganic fullerene-like (hollow onionlike) nanoparticles (IF) and nanotubes have attracted considerable interest in recent years, due to their unusual crystallographic morphology and their interesting physical properties. IF-MoS2 and nanotubes were first synthesized by a gas-phase reaction from MoO3 powder, This process consists of three steps: (1) evaporation of the MoO3 powder as molecular clusters; (2) condensation of the oxide clusters to give MoO3-x nanosize particles; (3) sulfidization of the suboxide nanoparticles to generate IF nanoparticles. The evaporation of MoO3 (step I) and the IF particle formation from the oxide nanoparticles (step 3) have been investigated already, while the mechanism for the suboxide nanoparticles formation (step 2) has not been studied before and is reported here. According to the present model, a partial reduction of the trioxide molecular clusters (3-5 molecules) leads to the formation of MoO3-x nanoparticles (5-300-nm particles size)-the precursor for IF-MoS2. A mathematical model, which takes into account the diffusion of the reactants into the reaction zone, the chemical reactions, and the boundary conditions obtained from the experiments, is established and solved. Based on the comprehensive understanding of the IF-MoS2 growth mechanism from MoO3 powder and the solution of the diffusion equations, a gas-phase reactor, which allowed reproducible preparation of a pure IF-MoS2 powder (50 mg per batch) with controllable sizes, is demonstrated.