In this work, compositional series of Gd-Nb-oxides and Gd-Ta-oxides are investigated as new potential high-k materials. Ultrathin oxide films in the range of 5-25 nm are deposited by spincoating. XRD and AFM show that the films are amorphous and smooth (RMS < 1 nm). Dielectric constants are derived from capacitance voltage (CV) measurements and EOT extraction. K values of the Gd-Nb-Ox series range from 13-16 while higher k values are obtained for the Gd-Ta-Ox series ranging from 15-19. For the Gd-Nb-Ox series, a higher k value is obtained for the most Gd-rich composition, while the opposite trend for the Gd-Ta-Ox series is noticed. Furthermore, the Gd-Ta-Ox shows the lowest leakages, as concluded from current-voltage (IV) measurements. A higher Gd content leads to decreased leakage, concurrent with increased band gaps.