The reaction between antitumor active dirhodium(II) tetraacetate and DL-methionine (HMet) was followed in aqueous solution and showed initially mixtures of 1:1 and 1:2 adducts [Rh-2(AcO)(4)(HMet)(H2O)] (AcO- = CH3COO-) and [Rh-2(AcO)(4)(HMet)(2)] formed at room temperature (RT), as evidenced by UV-vis spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Rh K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy confirmed methionine thioether binding to the axial positions of the Rh-2(AcO)(4) cage structure. With excess HMet at RT, stepwise displacement of the acetate groups was observed after some time using ESI-MS. Heating the solution to 40 degrees for 24 h accelerated the substitution reaction leading to stable dirhodium(II) species with two acetate ligands displaced by two methionine groups. The crystal structure of the purple [Rh-2(II)(AcO)(2)(D-Met)(L-Met)]center dot 6H(2)O compound obtained from the solution revealed tridentate coordination of the methionine ligands to the Rh(II) ions, with the thioether S atoms in equatorial positions. A minor amount of a light orange monomeric [Rh-III(Met)(2)](AcO) complex also formed in the solution was isolated by size exclusion chromatography and identified by ESI-MS. Crystals of [Rh-III(D-Met)(L-Met)]Cl center dot 3H(2)O were prepared by reacting RhCl3 and DL-HMet. The crystal structure showed tridentate binding of the methionine ligands to the Rh(III) ion in a trans-S,N,O arrangement.