Chemical preparation, crystal structure, calorimetric studies and spectroscopic investigation are given for a new organic cation dihydrogenomonophosphate [3,5-(CH3O)(2)C6H3NH3](2)(H2PO4)(2). This compound is triclinic P (1) over bar with the following unit cell parameters: a = 9.030(6) Angstrom, b = 16.124(5) Angstrom, c = 8.868(3) Angstrom, alpha = 75.04(3)degrees, beta = 110.71(4)degrees, gamma = 104.61(1)degrees, Z = 4, V = 1148.0(1) Angstrom(3), Z = 2 and rho(cal). = 1.454 g cm(-3). Crystal structure was solved and refined to R = 0.04, 2752 independent reflections. The atomic arrangement can be described as inorganic layers of H2PO4- anions parallel to y = 1/2 planes, between which are located the organic groups. Solid-state C-13 and P-31 MAS-NMR spectroscopies are in agreement with the X-ray structure. Ab initio calculations allow the attribution of the phosphorous and carbon signals to the independent crystallographic sites and to the various atoms of the organic groups. (C) 2003 Elsevier Ltd. All rights reserved.