The mol. and electronic structures of aq. I3- and I- ions have been investigated through ab initio mol. dynamics (MD) simulations and photoelectron (PE) spectroscopy of the iodine 4d core levels. Against the background of the theor. simulations, data from our I4d PE measurements are shown to contain evidence of coupled solute-solvent dynamics. The MD simulations reveal large amplitude fluctuations in the I-I distances, which couple to the collective rearrangement of the hydrogen bonding network around the I3- ion. Due to the high polarizability of the I3- ion, the asym. I-I vibration reaches partially dissocd. configurations, for which the electronic structure resembles that of I2 + I-. The charge localization in the I3- ion is found to be moderated by hydrogen-bonding. As seen in the PE spectrum, these soft mol. vibrations are important for the electronic properties of the I3- ion in soln. and may play an important role in its electrochem. function.