The thermal decomposition of yttrium hydrazidocarbonate trihydrate Y(N2H3COO)(3) . 3H(2)O has been studied in the temperature range 25-1400 degrees C. The influence of inert and oxidative atmospheres, e.g. argon, carbon dioxide and air, on the course of the thermal decomposition was determined. After dehydration to the anhydrous compound, the decomposition proceeds through several decomposition steps to yttrium oxide. In air, a pure high temperature C form of this oxide is isolated at a relatively low temperature, e.g. 708 degrees C. The same end product is obtained in carbon dioxide, albeit at higher temperatures, but in argon the yttrium oxide is mixed with amorphous carbon. The surface area and other particulate properties are substantially dependent on the heating programme of the sample.