The thermal stability of two single (K3R, K67R) and one double (K3R-K67R) mutants of Xenopus laevis B Cu,Zn superoxide dismutase has been studied to test Lys-->Arg substitution as an 'electrostatically conservative' strategy to increase protein stability, The K3R mutant displays an increased thermostability with respect to the wild-type enzyme, whilst a decreased stability was observed in the case of the K67R and K3R-K67R mutants, Concentration dependence of the apparent inactivation constant (k(app)) of the latter mutants, as compared to that of the wild type enzyme and K3R mutant, indicates that their higher sensitivity to heat inactivation is due to a perturbation of the dimer association. These results are confirmed also by fluorescence anisotropy measurements of the internal probe Tyr149. The possible role of Arg67 in perturbing the dimer dissociation equilibrium toward the monomeric form is discussed.