Thermal properties of (1,2-diaminoethane)bis(S-tyrosinato)cobalt(III) complex diastereomers are investigated by means of TG technique in 293-873 K temperature range. This is a first thermal study of [Co(S-aa)(2)en](+) (S-aa = anion of S-amino acid; en = 1,2-diaminoethane) complex diastereomers. It is shown that thermal decompositions of these complexes are multi-step degradation processes, which in some cases can be satisfactory separated into individual steps, depending on the molecular symmetry. It is proposed that the degradation processes occur with cleavage of the side chain of one S-tyrosinato ligand, followed by the cleavage of the second side chain by further heating. The proposed pathways of decomposition are in accordance with results of thermal degradation of noncoordinated S-tyrosine [F. Rodante, G. Marrosu, G. Catalani, Thermochim. Acta 194 (1992) 197-213]. In the case of diastereomers which crystallize with water molecules, the departure of both hygroscopic and crystal water occurs before. The corresponding kinetic and thermodynamic parameters of some processes were determined. (c) 2004 Elsevier B.V. All rights reserved.