Among the heavy ions, the Ce(III) ion, considered as a homologue of trivalent actinide ions, has interesting spectroscopic properties. A striking feature concerns the origin of the large red-shift in the luminescence spectrum in solution (357 nm) but not observed in crystal(312-330 nm). In the present work, we tried to find out a correlation between structural and spectroscopic properties of different cerium(III) hydrates, such as [Ce(H2O)n]3+ (n=8,9), [Ce(H2O)7Clm]3-m+ (m=1,2) and [Ce(H2O)6Cl2]+. The experimental results were supported by accurate quantum chemical electronic structure calculations on representative cluster models, extending thus the work of Kotzian and Rösch.1 This complementary approach enabled us to explore the potential energy landscape of the photo-dissociation reaction of a water molecule within the [Ce(H2O)9]3+* cation in the excited state, supposed to be responsible of the large red-shift. References [1] Kotzian, M.; Rösch, N. J. Phys. Chem. 1991, 96, 7288–7293.