Thin films with the composition 70mol% Na0.5Bi0.5TiO3+30mol% NaTaO3 were prepared by solgel synthesis and spin coating. The influence of the annealing temperature on the microstructural development and its further influence on the dielectric properties in the low- (kHzMHz) and microwave-frequency (15GHz) ranges were investigated. In the low-frequency range we observed that with an increasing annealing temperature from 550 degrees C to 650 degrees C the average grain size increased from 90 to 170nm, which led to an increase in the dielectric permittivity from 130 to 240. The temperature-stable dielectric properties were measured for thin films annealed at 650 degrees C in the temperature range between 25 degrees C and 150 degrees C. The thin films deposited on corundum substrates had a lower average grain size than those on Si/SiO2/TiO2/Pt substrates. The highest average grain size of 130nm was obtained for a thin film annealed at 600 degrees C, which displayed a dielectric permittivity of 130, measured at 15GHz.