The crystal structures of several novel alkylammonium phosphate salts were determined by single-crystal X-ray methods : cyclopentylammonium monohydrogen phosphate (CMP) ([C5H6NH3+](2)[HPO42-], monoclinic, space group P2(1)/n, Z = 4, a = 14.197(3) Angstrom, b = 5.992(1) Angstrom, 16.580(3) Angstrom, beta = 98.57(2)degrees); octylammonium dihydrogen phosphate (ODP) ([CH3(CH2)(7)NH3+] [H2PO4-], monoclinic, space group P2(1)/c, Z = 4, a = 4.554(1) Angstrom, b = 34.652(3) Angstrom, c = 7.328(1) Angstrom, beta = 90.66(1)degrees); and decylammonium dihydrogen phosphate (DDP) ([CH3(CH2)(9)NH3+] [H2PO4-], monoclinic, space group P2(1)/n, Z = 4, a = 7.358(2) Angstrom, b = 39.749(12) Angstrom, c = 9.131(4) Angstrom, beta = 90.45(2)degrees). Each are composed of anionic layers of hydrogen-bonded phosphate molecules, separated by charge-balancing alkylammonium cations. The long-chain alkylammoniums in ODP and DDP are interdigitated, while the cyclopentylammoniums in CMP define a bilayer-like arrangement. Large crystals were grown by recrystallization, but the materials were also observed to form in the corresponding solvothermal aluminophosphate synthesis mixtures. We have isolated phosphate salt compounds in aluminophosphate synthesis mixtures containing a variety of different alkylamines and suggest they are initially present in such experiments whenever the alkylamine is primary or secondary.