The betainic form of (imidazol-2-yl)phenylphosphinic acid hydrate.

Single crystals of the title compound, (imidazolium-2-yl)phenyl-phosphinate monohydrate, C(9)H(9)N(2)O(2)·H(2)O, were ob-tained from methanol/water after deprotection and oxidation of bis-(1-diethoxy-methyl-imidazol-2-yl)phenyl-phosphane. In the structure, several N-H⋯O and P-O⋯H-O hydrogen bonds are found. π-π inter-actions between the protonated imidazolyl rings [centroid-centroid distance = 3.977†(2)†à ] help to establish the crystal packing. The hydrate water mol-ecule builds hydrogen bridges to three mol-ecules of the phosphinic acid by the O and both H atoms.