Structure D 2000 Synthesis, Structure Refinement and Thermal Behavior of HEuP 2 O 7 ·4H 2 O. -Colorless prismatic crystals of the new title compound are prepared at room temperature from aqueous solutions of EuCl3 and H4P2O7. HEuP2O7·4H2O crystallizes in the monoclinic space group P21/n with Z = 4 (single crystal XRD). The crystal structure consists of corrugated layers of [HP2 ] groups alternating with layers of the Eu 3+ cations. Dehydration of the compound takes place between 361 and 1223 K to give a mixture of Eu4(P2O7)3 and Eu(PO3)3. -(ANNA-RABAH, Z.; CHEHIMI-MOUMEN*, F.; HASSEN-CHEHIMI, D. B.; TRABELSI-AYADI, M.; Solid State Sci. 8 (2006) 8, 932-939; Lab. Physico-Chim. Miner., Fac. Sci. Bizerte, 7021 Zarzouna, Tunisia; Eng.) -W. Pewestorf 43-006 O 3 -7