High-k dielectrics are intensively investigated as a replacement for SiO2 in integrated nanoelectronics. In particular, for the next generation of dynamic random access memory metal-insulator-metal capacitors, the Ta- and Nb-based oxides are among the most interesting candidates that display a relatively large dielectric constant with a bandgap larger than 3 eV. In this paper, we show that it is possible to modulate the dielectric response of Ta2O5 by admixing it with Nb2O5. The dynamical charges and dielectric constants of Ta2O5 and Nb2O5 for different crystal phases were calculated at the density functional theory level. The averaged dielectric constants range between 27 (38) and 42 (77) for Ta2O5 (Nb2O5) in the hexagonal and orthorhombic varieties. A mixed orthorhombic NbTaO5 composition exhibits a directionally averaged dielectric constant of 54, close to the arithmetic mean value of the binary species. The origins of the dielectric permeability are discussed and compared to experimental measurements. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3253583] All rights reserved.