The title compound, C(6)H(13)O(3)P, displays a crystallographic mirror plane. Bond lengths in the phosphonic acid moiety are P[double bond]O = 1.5557 (13) A and P[double bond]O = 1.5089 (18) A. The molecules are linked via intermolecular hydrogen bonding to form a one-dimensional chain of fused rings. There are no significant contacts between planes.