From fundamental ionic polarizability concepts, LaScO3 was projected as a candidate dielectric material for microwave application. The microwave dielectric properties of LaScO3 were obtained as the dielectric permittivity of 24, Q×f of 18,000 GHz, with a temperature coefficient of resonant frequency (TCF) of −69 ppm/°C. Solid solutions and polyphase assemblages were investigated in the LaScO3-TiO2 system as a function of TiO2 content, and correlated with the microwave dielectric properties. Orthorhombic perovskite LaScO3 transformed to tetragonal symmetry with Ti substitution on the B-site of LaScO3 owing to the ionic radius difference between Sc3+ and Ti4+. With this phase transition, the formation of A-site vacancies (VLa‴) resulted in the precipitation of a La deficient secondary phase. The substitution of Ti also affected the dielectric properties. The dielectric constant increased and TCF became less negative with increased TiO2 addition. However, the Q×f value decreased, due to the formation of the secondary phase. For the case of the 0.6LaScO3-0.4TiO2 composition, a dielectric K of 40, Q×f of 6,000 GHz and nearly zero TCF were obtained.