Solubility limitation of radionuclides by solid phases in aqueous environments is a key factor in performance assessment of radioactive waste repositories. Although the modeling of solubility limits is a standard procedure, results are often questionable because the basic data used are either irrelevant, inaccurate, or incomplete. This is illustrated by discussing the potential solubility limitation of Ni in sulfidic groundwaters, which is of some importance to the planned low- and intermediate-level radioactive waste repository at Wellenberg, Switzerland. Calculated solubility limits for Ni may be in error if a solubility-limiting sulfide mineral is chosen that is irrelevant for the considered geochemical conditions. Solubility data need to be carefully evaluated: In the case of millerite (NiS), the most likely Ni sulfide mineral to form, widely used solubility product constants turn out to be based on crude estimates only, and accurate solubility data are missing. The formation of Ni sulfide complexes may considerably enhance the solubility of Ni. Although reliable complexation constants for Ni sulfide complexes are missing, their neglect may result in a severe underestimation of Ni solubility in sulfidic environments, by analogy with Zn sulfide complexes whose complexation constants are reliably known.