Increasingly tightening regulations concerning the emissions of internal combustion engines have brought about new requirements for automobile exhaust catalysts. Among these requirements is the high thermal stability of the catalytically active materials. PtO2 especially is vulnerable at high operating temperatures of the catalysts. One solution to increase the thermal stability of the catalytically active material is to introduce PtO2 mixed with other metal oxides into the catalysts. For palladium catalysts this has been achieved by using neodymium, samarium or lanthanum. In this paper, BaNd2PtO5, BaEu2PtO5, BaSM2PtO5 and BaGd2PtO5 were synthesised by a novel, laboratory-scale method. The products were characterised by powder-XRD, XPS and TGA/DTA. The complete indexing of XRD powder data yielded the unit cell parameters and could be used to identify the solid phases. TGA/DTA experiments indicated that all phases were thermally stable up to 1250 degreesC. Based on the XPS measurements, the valence states of the elements were consistent with those of BaLn(2)PtO(5) (Ln: Nd, Eu, Sm, Gd). Catalytic activity was measured for the catalysts prepared from BaNd2PtO5 and BaEu2PtO5. The measurements were performed in both simulated gasoline exhaust gas and lean exhaust gas atmospheres, And the results indicated, that the lean exhaust gas catalysts operate in a temperature region (similar to270-300 degreesC) that cannot easily be achieved with conventional platinum catalysts.