The literature of the reactive flash sintering (RFS) in multi-component oxide systems nowadays consists of different types of binary phase diagrams, which were used here for analysis. Kinetic analysis of the reaction and simultaneous densification of the product phase via the liquid-film-assisted flash sintering is in agreement with a few seconds time scale of RFS. Capillarity and osmotic driving forces under the applied electric field are responsible for the ultrafast reaction and compound formation. Thermodynamic analyses of model systems with characteristic eutectic and peritectic reactions and congruent solidification show that all systems have to undergo local melting for the observed ultrafast reaction and the resultant microstructures. While the liquid film subjected to the external electric field has a transient nature, its homogenous composition is in the equilibrium state. The fragile nature of the major oxide melts imposes crystallization and growth during the cooling with partition coefficient of 1. Therefore, equilibrium phase diagrams can be used to predict the final product phases and microstructures according to the precursor powder composition and the flash conditions.