The solution chemistry of Pt(II) and Pd(II) cations has attracted experimental and theoretical attention because of their relevance in catalytic processes and the pharmacological activity of some derivatives in several cancer treatment. The usual square-planar ligand arrangement around these two transition-metal cations has drawn considerable attention to their axial coordination, because their reactivity seems to be intimately related to these regions. Here we propose a H/D isotopic substitution neutron scattering study of the Pd(II) aqua ion in acid aqueous solutions to gain deeper insight into the competition of water molecules and counterions to occupythis region. This experiment has been designed following the results obtained in a previous study where the unexpected presence of the perchlorate anion in the axial region was proven.