Reactions of cytotoxic platinum drugs with proteins are attracting growing attention for their relevant biological implications. We report here on the reactions of two cis-diphosphane platinum(II) dichlorides (namely cis-bis(trimethylphosphane) platinum(II) dichloride and cis-bis(triethylphosphane) platinum(II) dichloride) with horse heart cytochrome c (cyt c) monitored through advanced ESI MS methods coupled to enzymatic digestion. A remarkable selectivity in terms of adduct stoichiometry is highlighted and the specific metal binding sites are localised on the protein surface.