Synthetic amorphous aluminum phosphate (AlPO4) particles were treated with monohexyl, monooctyl, and monodecyl phosphates in acetone, and the resulting materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric and differential thermal analyses (TG/DTA), and adsorption of N-2. XRD patterns of the modified samples showed a strong diffraction peak and two weak ones below 2 theta = 15 degrees. The d-values of these peaks linearly increased with elongating of alkyl groups of the phosphates. After the treatment, particle size increased without formation of new particles. These results reveal that a multilayer alternating bimolecular layer of alkyl phosphates and aluminum phosphate phase is formed on the surface of AlPO4 particles by the modification with alkyl phosphates. The layered structure disappeared on outgassing above 300 degrees C, accompanying the removal of alkyl groups, formation of mesopores, and variation of surface structure.