It has recently been shown that 0.2CaTiO(3)-0.8(Li0.5Nd0.5) TiO3 (CTLNT) with the addition of 4 wt% Bi4B2O9 (BBO) as a liquid-phase sintering aid gives rise to exceptional microwave (MW) dielectric properties, (relative permittivity, epsilon(r) = 125, quality factor, Qf(0) and temperature coefficient of the resonant frequency, tau(f) = 4 ppm/degrees C) at reduced sintering temperatures (1200 degrees C). In this study, X-ray diffraction and scanning and transmission electron microscopy have been used to elucidate the mechanism by which MW dielectric properties are optimized. It is demonstrated that highly polarizable Bi3+ ions enter the perovskite lattice thereby increasing epsilon(r). The concomitant ex-solution of TiO2 as BBO concentration increases implies that ionic compensation occurs through the formation of V-Ti '''' in the perovskite lattice and infers that Bi3+ substitutes on the A-site for lower valence ionic species according to the equation: 4A(A)(x) + Ti-Ti(x) double right arrow 4Bi(A)(center dot) + V-Ti ''''. The residual boron rich material acts as a sintering aid with strongly -ve tau(f) that compensates for the + ve tau(f) of the perovskite matrix. This mechanism points to a new way of utilizing designer sintering aids in the development of microwave ceramics.