Determinations of the aqueous solubilities of kaolinite at pH 4, and of five smectite minerals in suspensions set between pH 5 and 8, were undertaken with mineral suspensions adjusted to approach equilibrium from over- and undersaturation. After 1,237 days, Dry Branch, Georgia kaolinite suspensions attained equilibrium solubility with respect to the kaolinite, for which K_eq = (2.72 ± 0.35) × 10⁷. The experimentally determined Gibbs free energy of formation (ΔG_f,298⁰) for the kaolinite is −3,789.51 ± 6.60 kj mol⁻¹. Equilibrium solubilities could not be determined for the smectites because the composition of the solution phase in the smectite suspensions appeared to be controlled by the formation of gibbsite or amorphous aluminum hydroxide and not by the smectites, preventing attempts to determine valid ΔG_f⁰ values for these complex aluminosilicate clay minerals. Reported solubility-based ΔG_f⁰ determinations for smectites and other variable composition aluminosilicate clay minerals are shown to be invalid because of experimental deficiencies and of conceptual flaws arising from the nature of the minerals themselves. Because of the variable composition of smectites and similar minerals, it is concluded that reliable equilibrium solubilities and solubility-derived ΔG_f⁰ values can neither be rigorously determined by conventional experimental procedures, nor applied in equilibriabased models of smectite-water interactions.