8 págs.; 6 figs. ; © 2015 American Chemical Society. The strong temperature dependence of the I−·(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process is determined using van’t Hoff analysis of key features that quantify the relative populations of Hbonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I−·(H2O)2 system sample the extended potential energy surface with increasing temperature. ; M.A.J. thanks the Air Force Office of Scientific Research (AFOSR) under Grant Number FA9550-13-1-0007 for the establishment of a temperature-controlled ion spectrometer at Yale. K.R.A. and H.K. acknowledge the support from the German Research Foundation within the Collaborative Research Center 1109. A.B.M. thanks the NSF for support under Grant Number CHE-1213347. M.J. and F.P. thank the NSF for support under Grant Number CHE-1305427 (“CCI: Center for Aerosol Impacts on Climate and the Environment”), while F.P. thanks the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation Grant Number ACI-1053575 (allocation TG-CHE110009). D.J.A.-A. acknowledge the support by MICINN Grant No. FIS2011-29596-C02-01 and to the FPI-MEC predoctoral fellowships Nos. BES-2012- 054209 and EEBB-I-14-08152. ; Peer Reviewed