Chemical preparation, crystal structure, calorimetric, and spectroscopic investigations are given for a new organic-cation dihydrogenomonophosphate, (4-C2H5C6H4NH3)H2PO4 in the solid state. This compound crystallizes in the orthorhombic space group Pbca with the following unit cell parameters: a = 8.286(3) Angstrom, b = 9.660(2) Angstrom, c = 24.876(4) Angstrom, Z = 8, V = 1991.2(7) Angstrom(3), and D-x = 1.442 g cm(-3). Crystal structure was solved with a final R = 0.054 for 3305 independent reflections. The atomic arrangement coaled described as H2PO4- layers between which are located the 4-ethylanilinium cations. (C) 2003 Elsevier Science Ltd. All rights reserved.