Kaolinite (K) was reacted with phenylphosphonic acid (PPA) in a water:acetone (1:1 V/V) solution (molar ratio 1 K:3 PPA), at 95 +/- 5 degrees C for different periods of time. Two different compounds (white powders) were identified but only one was isolated as a single phase and characterized by X-ray diffraction (powder method), thermal analysis (TG-DSC), Fourier-transformed infrared spectroscopy, and chemical analysis. The first compound showed a layer expansion of 7.86 Angstrom and the second, a layer expansion of 9.29 Angstrom, obtained as a decomposition product. Both compounds are consistent with the grafting of the phenylphosphonate group to the layers of kaolinite. The compounds have a formula of Al2Si2O5(OH)(HO3PPh)(3.0) . 2H(2)O as determined by thermal (water and total organic matter loss) and chemical (C content) analysis, respectively,