Abstract In this Letter, we derive a set of multifluid moment equations with and without internal and external fields from the collisional Boltzmann equation in a self-consistent manner. The new equations are mathematically closed and physically consistent with one free parameter, contained in a phenomenological closure for the collisional frequency and to be determined by experimental data. The new equations provide a theoretical foundation for a large fraction of phenomenological mix models. They contain all the physical terms, particularly the terms associated with the Reynolds stress due to both species interpenetrations and random chaotic motions. Under certain assumptions, the new model equations successfully reduce to the other mix models.