The salt N-benzylmethylammonium dihydrogenmonophosphate monohydrate is monoclinic with the following unit cell dimensions: a = 6.356(1)Angstrom, b = 8.385(7)Angstrom, c = 11.472(5)Angstrom, beta = 104.32(1)degrees, space group P2(1) with Z = 2. The structure consists of infinite parallel two-dimensional [IIO] planes built of mutually connected ions and water molecules by strong O-H ... O and N-H ... O hydrogen bonding. There are no contacts other than normal van der Waals interactions between the layers.