Chemical preparation, crystal structure, IR absorption and thermal analysis of a new cyclotetraphosphate [2-NH2-5-CH3C5H4N](4)P(4)O(12)center dot 6H(2)O are reported. This compound is triclinic P-1 with unit-cell parameters: a = 10.206(5), b = 11.778(1), c = 9.991(4) angstrom, a = 110.40(6), beta = 117.74(6), gamma = 86.41(3) V = 989.1(8) angstrom(3), Z = 1, D-x = 1.445 g cm(-3). The structure has been determined and refined to R = 0.034 and R-w = 0,044, using 3663 independent reflections. The P4O124- ring anions and water molecules form layers spreading around (a, b + c) planes via O-H center dot center dot center dot O hydrogen bonds. Between them are anchored 2-amino-5-methylpyridium cations, which establish H-bonds to interconnect the different adjacent layers and so contribute to the cohesion of the three-dimensional network. Tautomerization of (C6H9N2)(+) groups was evidenced in the present structure. (C) 2005 Elsevier Ltd. All rights reserved.