The chemical structure of aluminum phosphate binder, which is used as a sealant for thermally sprayed coatings, was studied. Powder X-ray diffraction (XRD) analysis showed that the binder was a mixture of several phases. None of these phases could be identified from the powder XRD data only. Electron diffraction revealed several distinct microcrystalline phases. The predominated phase was analyzed in more detail. The three-dimensional reciprocal lattice was reconstructed from a tilt series of electron-diffraction patterns that indicated that the main phase was the cyclohexaphosphate Al2P6O18. The similarity of the intensity distribution in the observed and simulated electron-diffraction patterns of Al2P6O18 confirmed this observation. The following unit-cell parameters were determined from the reconstructed reciprocal lattice: a = 8.8(7) Angstrom, b = 15.5(6) Angstrom, and c 6.3(9) Angstrom, beta = 108 degrees, and Z = 2, The reflection conditions indicated that the space group was P2(1)/a.