The effect of Al2O3 and Nb2O5 additions on the microstructure and thermal expansion behavior of tantalum M oxide has been studied. Both singly doped and co-doped compositions were examined. Compositions with 3 wt% Al2O3 (or greater), contained AITaO(4) as a second phase. Complete solubility was observed for Nb2O5 contents in the range 1-5 wt%. It was found that doping with Al2O3 results in a decrease of the coefficient of thermal expansion (CTE), such that increasing amounts of Al2O3 (1-5 wt%) cause a successive decrease in the CTE value. In the case of Nb2O5 additions, the result is an increase in the CTE of the tantalum (V) oxide. However, in the range 1-5 wt% Nb2O5, the CTE value is relatively insensitive to the amount of Nb2O5 added. Due to the countervailing influences of these two additive oxides, it was demonstrated that co-doping with Al2O3 and Nb2O5 is an effective strategy for tailoring the CTE of tantalum (V) oxide. (c) 2006 Springer Science + Business Media, Inc.