NH4[ErL(H2O)3].4H2O (I; H4L = pyromellitic acid) crystallizes in the monoclinic space group P21/n, with Z = 4, a 10.330(4), b 10.282(5), c 16.446(8) ? b 98.75(4)? R = 0.0364. The Er(III) ion is 8-coordinated, being bonded to the 5 O atoms in the carboxyl groups and 3 O atoms of H2O with dicapped trigonal antiprism and the crystal structure is a polymeric network along (001) face. The preparation of I is described.

Key words: AMMONIUM COMPOUND, CRYSTAL STRUCTURE DETERMINATION, BENZENECARBOXYLIC ACID P, HYDRATE, CHEMICAL BONDS, AQUO COMPLEX, ELECTRON DENSITY, MATRICES, LEAST SQUARE METHODS, POLYCARBOXYLIC ACID