Isotopic effects are discussed for protonated water in the Zundel form. In an 'isolated' deuterated Zundel ion, for a single H the center position has enhanced probability. Experimental studies focused on crystalline (H5O2+)(Br-) and its isotopic variants. For a 25% H/75% D mixture at 80 K, the probability of H occupying the central position of the Zundel ion exceeds statistical by a factor similar to 2.3. Diffusion Monte Carlo results indicate energy similar to 170 cm(-1) lower for the isotopomer with H at the ion center. DFT-based normal-mode analysis and on-the-fly molecular dynamics yield infrared spectra that are consistent with experiment. (c) 2005 Elseiver B.V. All rights reserved.