The microwave dielectric properties and microstructures of compounds in the solid solution series chiBaTiO(3)-(1-chi)La(Mg1/2- Til)03 (BTLMT) have been investigated. The structural phase transitions that occur as a function of x have been studied and are related to changes in the dielectric properties. For compounds where chi less than or equal to 0.1, X-ray diffraction (XRD) showed evidence of 1:1 ordering between Mg and Ti cations. For x less than or equal to 0.3, XRD and electron diffraction revealed that compounds were tilted in both antiphase and in-phase. However, for 0.3 < χ < 0.7, only antiphase tilting was present. The temperature coefficient of resonant frequency (tau(f)) vs the relative permittivity (epsilon(r) was linear until chi = 0.5 at which point in the solid solution the transition to a nontilted structure resulted in nonlinear behavior. tau(f), values close to zero (-2 ppm/degreesC were achieved at chi = 0.5 (epsilon(r) similar to 60), which had a quality factor (Q:f(o)) of 9600 GHz.