Compositions based on REZn1/2Ti1/2O3-CaTiO3 (RE=La and Nd), suitable for microwave (MW) applications have been developed by systematically doping the perovskite B site with Ta2O5, Al2O3, and MgO in order to tune the temperature coefficient of the resonant frequency (tau(f)) and improve the MW quality factor (Q x f(r)) Optimized compositions, 0.45La(Zn0.395Ti0.385Ta0.01Al0.21)O-3-0.55CaTiO(3) (LZT-CT) and 0.32Nd(Zn0.45Mg0.05Ti0.5)O-3-0.10NdAlO(3)-0.58CaTiO(3) (NZT-CT) were prepared by solid-state reaction and have been characterized. X-ray diffraction (XRD) and scanning electron microscopy revealed that both compositions were single phase with a bulk microstructure composed of equiaxed grains (similar to 10 mu m). Electron diffraction and XRD demonstrated that, at room temperature, LZT-CT and NZT-CT were orthorhombic with space group Pnma, consistent with an a-a-c+ tilt system. Diffraction contrast transmission electron microscopy revealed a complex domain structure consisting of ferroelastic and antiphase domain boundaries. LZT-CT and NZT-CT had zero tau(f), epsilon(r)=49 and 44, Q x f(r)=29 600 GHz (at 1.957 GHz), and 32 200 GHz (at 1.971 GHz), respectively.