A reversible crystal-to-crystal transformation of a new 2D lead(II) coordination polymer with the ligand 4-pyridinecarboxylic acid (4-Hpyc), [Pb(4-pyc)(Br)(H2O)](n) (1) to [Pb(4-pyc)(Br)](n) (2) by de-and rehydration. has been observed, and the structures of 1 and 2 have been 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 structural transformation occurs in the batch powder samples and leads to monophasic products.