A new three-dimensional europium-doped Yttrium(III) tricarboxylate, MIL-92 LT 1 or Y 1− x Eu x (H 2O) 2{C 6H 3 (CO 2) 3} ( x ∼ 0.03 ) (LT: Low Temperature) has been synthesised hydrothermally. Its three-dimensional structure, determined using X-ray powder diffraction data, is built-up from isolated eight coordinated Ln ( III ) monocapped square antiprisms ( Ln = Y , Eu) linked through carboxylate moieties. Its thermal behaviour, investigated using TGA and X-ray thermodiffractometry indicates that dehydration is irreversible giving the solid MIL-92 HT or Y 1− x Eu x {C 6H 3 (CO 2) 3} ( x ∼ 0.03 ) (HT: High Temperature). It induces a change in the connection mode of the carboxylate and a decrease in the coordination number of the rare-earth leading to an unusual octahedral environment for the rare-earth cation. After a structural analysis of the reconstructive phase transition, the optical properties of these solids have been investigated and show that dehydration leads to a strong increase in the optical output. Crystal data for MIL-92 LT : monoclinic space group C 2 / c (n° 15) with a = 16.428 ( 1 ) Å , b = 6.071 ( 1 ) Å , c = 20.404 ( 1 ) Å , β = 95.31 ( 3 ) ° and Z = 2 . Crystal data for MIL-92 HT : monoclinic space group C 2 / c (n° 15) with a = 17.390 ( 1 ) Å , b = 5.521 ( 1 ) Å , c = 19.487 ( 1 ) Å , β = 105.35 ( 2 ) ° and Z = 2 .