The reactions of carboplatin ([Pt(NH3)2(CBDCA)], in which CBDCA is cyclobutane 1,1, dicarboxylate) with L selenomethionine (L Se MetH) and L methionine (L MetH) were investigated using electrospray mass spectroscopy (ESMS) and 2D HSQC NMR spectroscopy. The dominant products for both reactions are bis chelated complexes [Pt(L Se Met)2] and [Pt(L Met)2]. These results are different from those observed for the reactions of cisplatin ([PtCl2(NH3)]) with L Se MetH or L MetH, in which the main products are mono chelated complexes [PtCl(NH3)(L Se Met)] or [PtCl(NH3)(L Met)]. This work provides insights into the differences between cisplatin and carboplatin in their reactivity and mechanism of reaction.