Ceramics of (1 − x)NaNbO3− (x)Na0.5Bi0.5TiO3 and (1 − x)NaNbO3 − (x)Sr0.5 NbO3 binary solid solutions exhibit strongly diffused permittivity maxima, when x exceeds a threshold value of about 0.15 and 0.1, respectively, but show weak frequency dispersion of permittivity and small frequency shift of the permittivity maximum temperature. Addition of LiNbO3 to these compositions increases the permittivity maximum temperature and greatly enhances the frequency dependence of both the permittivity maximum temperature and permittivity values.