A reversible crystal-to-crystal transformation of a 3D lead(II) coordination polymer with the ligand 4-pyridinecarboxylic acid (4-Hpyc), from [Pb(4-pyc)(2)(H(2)O)](n) (1) to [Pb(4-pyc)(2)](n) (2) by de- and re-hydration, has been observed and the structures of 1 and 2 were determined by single-crystal X-ray diffraction. The thermal stabilities of compounds 1 and 2 were studied by thermal, gravimetric and differential thermal analyses. Powder X-ray diffraction experiments showed that the phase transitions observed for the single crystals also occur in the batch powder samples and lead to mono phasic products.