The crystal structures of the potential antitumour agents monoethyl (alpha-anilinobenzyl)phosphonate, C(15)H(18)NO(3)P, (I), and its 4-azobenzene-substituted derivative monoethyl [alpha-[4-(phenyldiazenyl)anilino]benzyl]phosphonate, C(21)H(22)N(3)O(3)P, (II), are described. A zwitterionic form of (I) and a neutral molecular form of (II) are observed, which is fully in accordance with previously reported spectroscopic studies. In both structures, hydrogen bonding induces the formation of zigzag head-to-head double layers parallel to the crystallographic b axis.