2-Amino-pyrimidinium dihydrogen phosphate monohydrate.

In the title compound, C(4)H(6)N(3) (+)·H(2)O(4)P(-)·H(2)O, the pyrimidin-ium ring is essentially planar, with an r.m.s. deviation of 0.0016 Å. In the structure, pairs of symmetry-related anions are connected into centrosymmetric clusters via strong O-H⋯O hydrogen bonds forming six-membered rings with an R(2) (2)(6) motif. These clusters are inter-connected via water mol-ecules through OW-H⋯O hydrogen bonds, building an infinite layer parallel to the ab plane. Moreover, infinite chains of 2-amino-pyrimidinium cations spread along the a-axis direction. These chains are connected to the inorganic layer through N-H⋯O, C-H⋯O and C-H⋯N hydrogen bonds, which, together with electrostatic and van der Waals inter-actions, contribute to the cohesion and stability of the network in the crystal structure.