Mid infrared, far-infrared and Raman spectroscopic studies, combined with force field analyses, were performed for the hydrated platinum(II) and palladium(II) ions and cis-diammineplatinum(II) complex in acidic aqueous solutions. Simplified Density Functional Theory (DFT) calculations were made for the equatorial plane of the platinum complexes. Careful subtraction of solvent spectra allowed a number of 'internal' modes of coordinated H 2 O and NH 3 to be determined as weak residual bands. The [Pt(OH 2 ) 6 ] 2+ and [cis-Pt(NH 3 ) 2 (OH 2 ) 4 ] 2+ complexes were found to be six-coordinated with four ligands strongly bound in an equatorial plane. The assignments of the vibrational modes in the equatorial plane could be performed on the basis of the experimental observations and by comparison with metal-ligand vibrations of square-planar complexes, aided by normal coordinate calculations. For the weakly coordinated axial aqua ligands, the low wavenumber range and the polarizibility properties allowed the assignments of the bands at about 365 and 325 cm -1 to the stretching modes of one short and one longer Pt -O* bound to axial aqua ligands, respectively. A similar picture with even less strongly bound axial water molecules emerges from Raman spectroscopy data for the hydrated palladium(II) ion, [Pd(OH 2 ) 6 ] 2+ . The results are consistent with a description of the [Pt(OH 2 ) 6 ] 2+ and [Pd(OH 2 ) 6 ] 2+ aqua ions in C 4v symmetry, and with the [cis-Pt(NH 3 ) 2 (OH 2 ) 4 ] 2+ complex in the C s point group, and also in qualitative agreement with the structures devised from previous extended X-ray absorption fine structure (EXAFS) measurements.