Capillary electrophoresis-electrospray ionization-mass spectrometry (CE-ESI-MS) has been used for investigating the influence of the sulfur containing amino acid L-methionine (L-Met) on the binding behavior of oxaliplatin (trans-R,R-diaminocyclohexane(oxalato)platinum(II)) to 5'-GMR L-Methionine competes with 5'-GMP for the platinum binding site and forms as well as 5'-GMP adducts with oxaliplatin. The formation of the prognosed complexes [Pt(DACH)(L-Met-S,N)](+) and [Pt(DACH)(5'-GMp)(2)](2-) (DACH = 1,2-diaminocyclohexane) could be proved directly by using CE-ESI-MS. Furthermore, we could now bring forward proofs, that the coordination of 5'-GMP with oxaliplatin is inhibited by L-methionine and could show, that the 5'-GMP ligands of the [Pt(DACH) (5'-GMp)(2)](2-) complex can be replaced slowly by L-methionine whereas methionine can not be replaced by GMP.