In this chapter we construct multicomponent chemical reaction models of how an aqueous fluid might react with the minerals it contacts, according to kinetic rate laws. We show that the fluid can be in equilibrium or disequilibrium with respect to the minerals, and how the fluid chemistry can approach an apparent equilibrium that is in fact a steady state, rather than a thermodynamic equilibrium. We further construct example calculations that demonstrate the basis of Ostwald’s step rule and the nature of the incongruent dissolution of albite feldspar.