Solid solutions were obtained on the basis of (Na,Li)NbO3 binary system, modified by addition of minor amount (not greater than 5 wt%) of various elements' monoxides, including their combinations. This study focuses on the peculiar features of their crystalline structure, grain structure, and thermal-frequency behavior of relative permittivity. The modifications by simple Mn and Cu oxides demonstrate a retention of crystalline structure of initial solid solution. Addition of NiO leads to transformation of phase state, which is stipulated by high stereochemistry activity and emission ability of Ni(II). A noticeable variation of phase state occurs also due to the combined modification by the above-mentioned oxides and Bi2O3+Fe2O3 ones. It is established that high-temperature anomalies of dielectric properties are related to partial reduction of niobium. It was also shown that the given modified lead-free solid solutions are characterized with improved piezoelectric, ferroelastic, and mechanical properties, and they can be applied in the field of microwave engineering.