The intercalation of hexacyanoferrate(III) in chloride-hydrotalcite was studied as a function of the composition of the equilibrated aqueous phase, the relevant solution variable being the ratio (C(Fe(CN)(6)(3-))(1/3)/C(Cl)-. The isotherm shows that the exchange is markedly nonideal and reveals the segregation of two solid phases of fixed compositions at (C(Fe(CN)(6)(3-))(1/3)/C(Cl)- = 17.1 M(-2/3). This is the first case of solid phase segregation during anion exchange in hydrotalcite systems diagnosed from thermodynamic measurements. In agreement, PXRD patterns of the partially exchanged solids indicate that these phases, with distinct basal spacings, coexist in the range 0.21 < or = x < or = 0.86. The observed behavior is accounted for by classical thermodynamic formalisms of ion exchange. The limitations of other representations commonly used to describe anion exchange in layered double hydroxide systems are stressed. A rationale of the structural changes accompanying the exchange is also offered.