The first experimentally determined hydration energies of Cd2+(H2O)n complexes, n = 4-11, are measured using threshold collision-induced dissociation in a guided-ion-beam tandem mass spectrometer coupled with an electrospray-ionization source. Kinetic-energy-dependent cross-sections are obtained and analyzed to yield 0 K thresholds for losing one water ligand. The threshold measurements are converted to 298 K values to give the hydration enthalpies and free energies for sequentially losing one water. Trends in these values and calculations at the MP2(full)/SD/6-311+G(2d,2p)//B3LYP/SD/6-311+G(d,p) level are consistent with the inner-solvent shell of Cd2+ being six waters. (C) 2009 Elsevier B. V. All rights reserved.