The thermal decomposition of the Pt(II) complexes with cyclobutane-and cycloheptanespiro-5'-hydantoins were studied by TG and DTA techniques. The Pt(II) complex with cyclobutanespiro-5'hydantoin (PtCBH) was stable up to 115 degrees C (388 K) and Pt(II) complex with cycloheptanespiro-5'-hydantoin (PtCHTH) was stable up to 150 degrees C (423 K). After the thermal decomposition of PtCBH the solid residue was platinum, while the decomposition of PtCHTH gave a mixture of platinum carbides (PtC2, Pt2C3).