Substituted hetarylhydrazones are important heterocyclic compounds, which have in their structure few sites capable of complexation of metals giving potentially cytotoxic complexes [1]. Thermal analysis is convenient and useful method for investigation of the chemical content of the complexes with metals (additionally to spectral methods such as FTIR, UV-Vis, and XRD). The titled complex was obtained by heating of the ligand, 5-phenyl-3-[2-(thiazol-2yl)hydrazono]furan-2(3H)-one (HL), and Cu(OAc)2 · 2 H2O in acetonitrile. Hetarylhydrazones exist as few tautomeric forms dependently from pH of the medium and other factors (the presence of ortho-substituents may give the possibility of existence the intramolecular hydrogen bonds etc.). Due to their polydenticity they can coordinate metals in different ways. The simultaneous thermogravimetric (TG) and differential thermal analysis (DTA) measurements of the obtained complex were recorded from RT to 1000 °C on a OD-103 derivatograph (MOM, Hungary) analyzer. The experiments were carried out in air atmosphere at a ramping rate of 10 °C per min with 100 mg of the sample. Calculated from DTA analysis molecular mass of the complex revealed that the Cu and L are in ratio 1:1. Two molecules of acetonitrile are also present. These data are consistent with the elemental analysis. O O Ph N N S N Cu N C C N O OH Ph N N S N Cu(OAc)2 2 H2O MeCN In the molecule of the L there is the only one proton capable of migration and substitution. It is obvious that during the reaction occurs a reduction of Cu(II) by hydrazones giving rise the monovalent copper species. This along with the data [2], where it was reported about the redox process during the complexation, allow us to propose the structure of the complex as the Cu(I) species Cu(L)(MeCN)2 were the ligands are bidentate.