In general, all the platinum metals form stable, very thin oxide films in air at room temperature and the thickness of each fill shows little change until about 400°C, above which it increases with temperature [15-17]. At a temperature above 400°C the oxide fill rapidly reaches a maximum thickness and, thereafter, its thickness and weight remain nearly constant [16]. However, each of the solid oxides will begin to dissociate at a specific temperature and at significantly higher temper- atures the oxide exists only as a gas. The dissoci- ation temperature is about 400°C for PtO2 [15], about 870°C for PdO [17] and above 1000°C for the oxides of Ir and Rh, depending on the partial pressure of 02, etc. According to Chaston [16] the solid oxides for Ir and Rh are IrO2 and RhO2, and the stable gas phase for iridium oxide is probably IrO3. On the other hand, Carol and Mann [18] claim the predominant oxide of Rh when solid is Rh20 a.