Permittivity, dielectric loss tangent, radial coupling coefficient, and radial frequency constant have been measured as a function of composition for pressure‐sintered Li.rNa1‐xNbO3in the range 0.015≤x≤0.15. An anomaly in each of these properties when plotted as a function of composition is associated with a structural transition between 2 ferroelectric phases; this transition lies near room temperature for compositions where x∼0.12. Compositions near this phase boundary exhibit room‐temperature properties which are of interest for high‐frequency filter