In the present work quantum mechanical methods were used to calculate the rate constants for the first step of the aquation of a set of 4-substituted (o-phenylenediamine)platinum(II) dichloride derivatives containing electron-donating and withdrawing substituents at the 4-position of the aromatic ring. A linear free energy relationship was obtained for log(k(X)/k(H)), k being the rate constant for the first step of hydrolysis, and the electronic Hammett constants sigma(m) and sigma(p). The results showed that electron-donating groups promote the hydrolysis reaction. The quantitative models described here may be useful for the rational design of new, less mutagenic drugs based on platinum complexes.