Three new barium 4-carboxyphenylphosphonates with formulae Ba(HOOCC 6 H 4 PO 3 H) 2 , BaH(OOCC 6 H 4 PO 3 ), and Ba 3 (OOCC 6 H 4 PO 3 ) 2 ·2H 2 O were prepared and characterized by elemental analysis, thermogravimetric analysis, X-ray powder diffraction, energy-dispersive X-ray microanalysis, and infrared spectroscopy. It was found that depending on the acidity of the reaction medium, Ba(HOOCC 6 H 4 PO 3 H) 2 can be converted to Ba 3 (OOCC 6 H 4 PO 3 ) 2 ·2H 2 O and vice versa. As an intermediate in these reactions, BaH(OOCC 6 H 4 PO 3 ) is formed.The infrared spectra of these compounds were compared with analogous strontium compounds prepared and described previously and the position of the acidic hydrogen atom in BaH(OOCC 6 H 4 PO 3 ) is deduced from the spectra.The structure of Ba(HOOCC 6 H 4 PO 3 H) 2 was solved from X-ray powder diffraction data by an ab initio method using a FOX program with subsequent Rietveld refinement in the FullProf program. The compound is monoclinic, space group C2/c (No. 15), a=49.382(1), b=5.5196(1), c=8.4977(2)Å, β=127.52(1)°, and Z=4. This compound has a layered structure built up from CPO 3 phosphonate tetrahedra and BaO 8 distorted tetragonal antiprisms and thus forming layers in the bc plane with organic groups pointing to the interlayer space.