This paper reports the first systematic investigation of the selectivity of organic guest intercalation into hydroxy double salts. The organic guests 1,2- and 1,4-benzenedicarboxylate (1,2- and 1,4-BDC) have been incorporated into a range of hydroxy double salts by anion exchange. Intercalates of [Zn₅(OH)₈](NO₃)₂·yH₂O, [Zn₅(OH)₈]Cl₂·yH₂O, [Zn₅(OH)₈](CH₃COO)₂·yH₂O, [Zn₃Ni₂(OH)₈](NO₃)₂·yH₂O, and [Zn_3.8Co_1.2(OH)₈](NO₃)₂·yH₂O were prepared, and the resultant materials fully characterized. 1,5- and 2,6-Napthalanedisulfonate (1,5- and 2,6-NDS) were also successfully intercalated into the [Zn₅(OH)₈](NO₃)₂·yH₂O material. It was found that the initial anions are almost completely replaced by the new organic guests in the majority of cases. Selected reactions were investigated by in situ X-ray diffraction, and the reactions observed to proceed directly from the host to the product largely under nucleation control. No intermediate phases were detected. The competitive intercalation of isomeric pairs of guest anions was explored, and very high degrees of preferential intercalation found for 1,4- over 1,2-BDC into all hydroxy double salts (HDSs) studied, and for 2,6- over 1,5-NDS into [Zn₅(OH)₈](NO₃)₂·yH₂O. The selectivity was found to be largely invariant with reaction time, reaction temperature, solvent system, and guest concentration. It was also observed to be very similar across all the HDSs explored. In situ diffraction and NMR demonstrate that the selective intercalation is a thermodynamically controlled phenomenon. It is therefore suggested that preferential intercalation is governed in the main by the strength of the interactions between the HDS layers and the guest ions.