In the previous chapter, we considered single-component systems for which there was a single density, $$\rho$$, a single velocity, $$\mathbf{v}$$, and no chemical reactions. In multicomponent systems we must deal with the density of individual species and this leads to the characterization of systems in terms of species mass densities and species molar concentrations, in addition to mass fractions and mole fractions. Not only must we characterize the composition of multicomponent systems, but we must also consider the fact that different molecular species move at different velocities. This leads us to the concept of the species velocity which plays a dominant role in the detailed study of separation and purification processes and in the analysis of chemical reactors. In this chapter we will discuss the concept of the species velocity and then illustrate how a certain class of macroscopic mass balance problems can be solved without dealing directly with this important aspect of multicomponent systems. While chemical reactions represent an essential feature of multicomponent systems, we will delay a thorough discussion of that matter until Chapter 6.