Solid solutions in the Ag(NbxTa1−x)O3 (where 0 ≤ × ≤ 1) system exhibit excellent dielectric properties at microwave frequency including high dielectric constant (200<k<400), low loss (tanΔ ∼ 0.004) and a composition controlled temperature coefficient of capacitance (TCC). For Ta-rich and Nb-rich solid solutions, the TCC values are negative and positive, respectively, and two-phase mixtures provided an average TCC close to zero. Series, parallel, and logarithmic mixing rules were applied to predict dielectric constant and TCC of polyphase assemblages (45wt%Ag(Nb0.65Ta0.35)O3+55wt% Ag(Nb0.35Ta0.65)O3) yielded an average dielectric constant of 450 and a TCC of 180ppm/°C. Microstructure analysis revealed that a CuO-rich liquid remains at the grain boundary and transmission electron microscopy shows that the CuO resides at triple points. Ag(Nb x Ta1−x )O3 ceramics were successfully integrated into LTCC for embedded capacitors. The addition of CuO lowered the sintering temperature to below 900°C and a low TCC was maintained for fine grained microstructures.