Abstract While the study of ion partitioning in solid solution-aqueous solution (SS–AS) systems has garnered scientific attention for decades, the terminologies used in ion-partitioning studies are often inconsistent. Moreover, the equilibrium thermodynamics underlying the partitioning process is often disregarded or misunderstood, although this background is indispensable in interpreting effective (non-equilibrium) distribution of substituting ions between minerals and their growth environments. Here, after an introduction to the different terminologies and definitions, the equilibrium thermodynamics of ion partitioning is reviewed for the simplest case of solid solutions with isostructural end-members and one mixing position per formula unit. Variations of the equilibrium distribution coefficients with changes in the background solution or in the composition of the solid as well as the methods to determine equilibrium values are discussed in depth. The simplifications when one of the substituting ions occurs in trace amounts (dilute solid solutions) and the effect of temperature are introduced in a later tion, followed by a discussion of the thermodynamic treatment of more complex SS–AS systems: ion partitioning involving solids with more than one mixing position per formula unit, solid solutions with non-isostructural end-members, heterovalent substitutions, etc . Finally, the concept of supersaturation in SS–AS systems is introduced as a fundamental parameter to interpret non-equilibrium partitioning.