The potassium salt of phthalhydroxamic acid (PHA) (alternatively named o-carboxybenzohydroxamic acid) was synthesized and its coordination behaviour with Mn(II), Co(II), Ni(II), Zn(II) and Cd(II) was studied. The hydroxamate ligand reacts with the transition metal ions to form polymeric complexes. The ligand is coordinated to the metal ions through the deprotonated O and carbonyl O of the hydroxamate group, and the carboxylate group, behaving as a tetradentate ligand. KPHA and its solid metal complexes were characterized by elemental analyses, electronic and IR spectroscopy, and magnetic moment measurements. The thermal decomposition behaviour of the ligand and the metal complexes was also studied. On the basis of the experimental data, structures of the complexes are proposed.