Flame spray pyrolysis has emerged as a promising technology to synthesize metal-oxide nanomaterials in a scalable way. However, numerical simulation of the reactive flow system is challenging due to the complex phase transitions and homo- and heterogeneous reactions during the transformation from droplets to nanoparticles. In the current study, we develop a theoretical single-droplet model to describe multicomponent droplet combustion with the consideration of precursor reaction and particle formation routes. The model is first validated by pure-component and multicomponent droplet combustion experiments and then utilized to describe precursor decomposition and solid-phase formation inside the droplet. Microexplosions are significantly enhanced by the less-volatile precursor species that accumulates at the droplet surface, forms a viscous layer, and traps the volatile species inside the droplet. Large liquid Lewis numbers and small precursor decomposition rates can significantly accelerate droplet microexplosion. The nanoparticle formation via either the droplet-to-particle or the gas-to-particle conversion route is classified by the ratio of precursor consumption through evaporation into the gas phase to that by decomposition within the liquid phase. The modeling results indicate that more nanoparticles can be generated through the gas-to-particle conversion route for smaller droplets in a hotter environment. The single-droplet model established in the current study can guide precursor and process design, and it can also be coupled with multiphase turbulent flame simulations of the whole flame spray pyrolysis process.