The properties of hydrogen ions in aqueous solution are governed by the ability of water to incorporate ions in a dynamical hydrogen bond network, characterized by a structural variability that has complicated the development of a consistent molecular level description of H+(aa). Isolated protonated water clusters, H+(H2O)(n), serve as finite model system for H+(aq), which are amenable to highly sensitive and selective gas phase spectroscopic techniques. Here, we isolate and assign the infrared (IR) signatures of the Zundel-type and Eigen-type isomers of H+(H2O)(6), the smallest protonated water cluster for which both of these characteristic binding motifs coexist, down into the terahertz spectral region. We use isome-selective double-resonance population labeling spectroscopy on messenger-tagged H+(H2O)(6)center dot H-2 complexes from 260 to 3900 cm(-1). Ab initio molecular dynamics calculations qualitatively recover the IR spectra of the two isomers and allow attributing the increased width of IR bands associated with H-bonded moieties to anharmonicities rather than exicited state lifetime broadening. Characteristic hydrogen-bond stretching bands are observed below 400 cm(-1).