The crystal structure of triphenylarsine oxide monohydrate has been determined from three-dimensional photographic data. The crystals are monoclinic, space group P2₁/c, with four molecules of Ph₃AsO·H₂O in a unit cell of dimensions a= 11·14, b= 16·65, c= 11·41 Å, β= 130·7°. The structure was solved by the heavy-atom method and refined by full-matrix and block-diagonal least-squares calculations. R is 0·114 for 2187 independent reflexions. The crystal structure comprises discrete centrosymmetric dimers with each water molecule hydrogen bonded to two oxygen atoms of triphenylarsine oxide molecules and vice-versa. The O(H)⋯ O distances are 2·78 and 2·81 Å. The triphenylarsine oxide molecule possesses no symmetry because of unequal rotations of the phenyl rings atound the C–As bonds. Some molecular dimensions are AsO 1·644 ± 0·007, As–C 1·907 ± 0·009 Å, C–As–C 108·0, and C–As–O 110·9°.