The substitution behavior of cis-Pt(NH3)2(1-MeU)2, cis-Pt(NH3)2(1-MeU)Cl and cis-[Pt(NH3)2(1-MeU)H2O]+ (1-MeU = 1-methyluracil anion, C5H5N2O2-) was studied in detail as a function of entering nucleophile concentration, pH, temperature, and pressure. The reactivity of these species is controlled by the lability of the aqua complex. Protonation of the exocyclic O(4) atom of the 1-methyluracil ligand is essential in order to increase the lability of the bis(methyluracilato) complex. Solvolysis is the rate-determining step for substitution reactions of the latter complex, for which k = (2.40 +/- 0.06) x 10(5) s-1 at 60-degrees-C and pH = 3, DELTAH(double-dagger) = 79 +/- 1 kJ mol-1, DELTAS(double-dagger) = -98 +/- 11 JK-1 mol-1, and DELTAV(double-dagger) = -5.6 +/- 0.6 cm3 mol-1. All the reported rate and activation parameters support the operation of an associative substitution mechanism. The results are discussed in reference to data reported in the literature for related systems.