The chemical preparation, crystal structure, and the results of calorimetric and spectroscopic investigations are given for a new organic cation diphosphate, (m-CH3C6H4NH3)(2)H2P2O7. This compound is monoclinic C2/c with the following unit-cell parameters: a = 29.752(4), b = 8.477(2), c = 7.738(6) Angstrom, beta = 104.23(4)degrees, Z = 4, V = 1892(1) Angstrom (3), and D-x = 1.377 g(.)cm(-3). The crystal structure has been determined and refined to R = 0.035, using 2918 independent reflections. The structure can be described as a stacking of a thick layer centered on the (b,c) plane. These planes are built of corrugated ribbons of H2P2O72- groups running parallel to the c axis and linked by hydrogen bonds formed by the organic groups (m-CH3C6H4NH3)(+). The H2P2O72- anions are located around the twofold axis.