Ceramics of the system LiNb O -NaNbO 3 were prepared from sol-gel derived powders in the whole range of compositions and structurally characterized by X-ray diffraction. The impedance of thin ceramic disks was measured as a function of the temperature from 1 kHz to 15 MHz up to 800°C. An automatic iterative procedure was used to calculate the electromechanical coupling factor of the thickness mode resonance from impedance data. The solid solution limits of Na in LiNbO 3 and Li in NaNbO 3 were found to be 7 mole% and 15 mole%. As a consequence the materials whose nominal composition is Li 1 - y Na x NbO 3 with 0.07 ≤ X ≤ 0.85 contain mixtures of the two limit solid solutions. For ceramics with Na content exceeding 10 mole%, two dielectric anomalies at 300 and 400-450°C are observed as local maximums in the real part of the dielectric permittivity when measurements are made while heating the sample. These are more enhanced as the Na content increases. There is a change of sign in the temperature coefficient of the conductivity of the ceramics giving place to a minimum at low temperature, 100-150°C, which is not present at high frequency (> 5 MHz). There is a shift in the frequency number of the thickness mode of resonance towards higher values together with a minimum in the electromechanical coupling coefficient at about 350°C. For high Na content ceramics (> 90 mole%) an irreversible decrease to zero of the coupling coefficient is observed instead. The second anomaly in the real part of the dielectric permittivity and the anomalies in the electromechanical behaviour are related with the antiferroelectric-orthorhombic to paraelectric-orthorhombic transition reported for NaNbO 3 at 370°C. The first anomaly in the real part of the dielectric permittivity and the change in the temperature coefficient of the conductivity are linked with a change in the conductivity mechanisms of localized charge carriers, allowed by a certain degree of structural disorder caused by the Na + substitution of the smaller Li + ion, or viceversa, as reported for similar systems.