Aqua ligands in mixed aqua/nucleobase metal complexes are potential sites of acid-base catalysis and/or, when present as hydroxo ligands, can directly be involved in hydrolysis reactions. pK(a) values of close to 7 are consequently of particular interest and potential significance. Here we report on the differential acidity of aqua complexes in model nucleobase (nb) complexes of cis- and trans-[Pt(NH3)(2) (nb)(H2O)](n+) and discuss reasons as to why the nb in cis complexes influences the pK(a) (pK(a) 4.8-7.0), whereas in trans complexes the pK(a) values are rather constant (pK(a) similar to 5.2-5.3). The results of DFT calculations of a series of mono(nucleobase) complexes derived from cis-Pt(NH3)(2) are critically examined with regard to the role of exocyclic groups of nucleobases in stabilizing aqua/hydroxo ligands through intracomplex hydrogen bond formation. This applies in particular to the exocyclic amino groups of nucleobases, for which gas-phase calculations suggest that they may act as H bond acceptors in certain cases, yet in the condensed phase this appears not to be the case.