document: There is still some debate about the crystal structure and composition of the fine oxides found in ODS steels and a number of different phases have been both proposed and identified. A complete characterisation of the oxide particles, including crystal structure and composition, is needed as different phases and chemical variants of a single structure have been shown to respond differently to high temperatures and irradiation. Ribis and de Carlan [6] have studied the coarsening characteristics of Y2O3 and Y2Ti2O7 oxides at high temperatures. They show that the increase in particle size is greater for the non-Ti containing phase. Similarly, Ratti et al. [9], although they do not allude to specific oxide phases, have shown that small Ti additions to an 18%Cr ODS alloy dramatically reduces the coarsening rates of dispersoids when compared to an equivalent alloy without titanium. For example, Ribis indicates that coarsening rates may be controlled by interfacial energy between the secondary phase particles and the matrix; he points out that the resistance to coarsening observed in the Y, Ti, O system is probably the result of a very low interface energy and this would differ from one phase to another. Whittle et al. [10] have shown that pyrochlore and structures closely related to the pyrochlore structure respond in different ways to irradiation. They revealed that oxide structure and variations in composition can affect their ability to withstand and recover from irradiation induced damage.