Phenylphosphonic acid was thermally intercalated into both the host Mg-Al-CO3 layered double hydroxide (LDH) and its corresponding oxide material (LDO) formed by the thermal decomposition of the host LDH via a novel slow heating method. In the thermal intercalation process, the host and guest reactants are carefully heated together to a temperature just above the melting point of the guest. The resulting intercalation compounds were studied by a variety of analytical techniques. Powder X-ray diffraction (PXRD) indicated that the reaction products were all polyphasic microcrystalline materials and suggested that arrangement of the phenylphosphonate anions between the LDH and LDO layers was similar to that exhibited by zirconium (IV) bis(benzenephosphonate), alpha-Zr(C6H5PO3)(2) formed via a topotactic reaction. Fourier-transformed infrared (FTIR) was inconclusive, while thermal analysis of the reaction mixture clearly showed the reaction taking place. Al-27, C-13 and P-31 solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) indicated the presence of the anion of phenylphosphonic acid together with some free/unintercalated acid. Collectively, these techniques suggested that the phosphonic acid was intercalated between the layers of both LDH and LDO hosts by way of an acid-base condensation reaction.