Six aluminum phenylphosphonates have been synthesized depending upon the synthetic conditions : Al-2(O-3-PC6H5)(3). 2H(2)O (I), Al-2(O3PC6H5)(3) (II), alpha-Al(HO3PC6H5)(O3PC6H5). H2O (III), beta-Al(HO3PC6H5)(O3PC6H5). H2O (IV), Al(HO3PC6H5)(3). H2O (V), and Al(OH)(O3PC6H5) (VI). Thermal analysis, X-ray powder thermodiffractometry, IR spectroscopy, and Al-27 and P-31 MAS NMR data have been obtained to study the structure and thermal stability of these materials. III crystallizes in the orthorhombic system, space group Pbca, with a 9.7952(1) Angstrom, b = 29.3878(4) Angstrom, c = 9.3537(3) Angstrom, and Z = 8. The structure : was solved ab initio, from synchrotron data (lambda approximate to 0.4 Angstrom), using direct methods, and refined by Rietveld methods. The final agreement factors were R-wP = 6.73%, R-P = 5.24%, and RF = 6.8%. The compound is layered with the aluminum atoms in an octahedral environment of oxygens and two crystallographically independent phosphonate groups, one being protonated. The powder patterns of V and VI have been indexed, and the experimental observations are consistent with layered structures. The unit cell of V contains one : octahedral site for Al and three tetrahedral sites for P. Phosphonate I seems to have a three-dimensional tubular structure with aluminum atoms in both octahedral and tetrahedral environments and phosphorus atoms in three different types of sites.