Abstract Alpha-layered zirconium carboxy n-alkyl phosphonates, Zr(O 3P(CH 2) n COOH) 2 with n=1−5 have been prepared and characterized by X-ray diffraction, TG-DTA, DSC and ac-conductivity measurements. The interlayer distance of the phosphonates increases almost linearly with increasing n from 11.1 to 18.7 Å. Compounds with n=2, 4, 5 undergo a reversible phase transition at 165, 95 and 215°C, associated with a weak endothermic effect and with a discontinous increase of the interlayer distance of 1.0, 1.3 and 0.8 Å respectively. Structural considerations indicated a different arrangement of the terminal-COOH groups in the interlayer region as a consequence of the odd or even number of methylene groups in the alkyl chain. ac-conductivity measurements, performed on pressed pellets by the impedance method, in fact showed that compounds with n=2 and 4 have a conductivity much lower than that of compounds with n=1, 3, 5. The conductivity of these latter is 2×10 -9, 1.6×10 -8 and 1.4×10 -7S cm -1 at 200°C, with activation energies of 77, 104 and 132 kJ mol -1 respectively. Finally the high frequency limit capacitance of the pellets made up of compounds with n=2 and 4 was found to change significantly at the phase transition temperature indicating a change in dielectric properties of these compounds.