Niobate glasses and ceramics [(x Nb2O5 (0.5 - x) P2O5) 0.5 Li2O] were prepared by the melt-quenching method and analysed by means of infrared spectroscopy, X-ray powder diffraction and complex impedance methods. LiNbO3 crystals were detected for high niobium content samples. The study of the complex dielectric function of the LiNbO3 ceramics revealed a very broad acoustic resonance mode around 3 MHz. This resonance was associated to the thickness mode of the LiNbO3 plate ceramic resonator. Electrical poling was performed at 300 degrees C for 6 hours under an applied dc potential of 1 kV. Strong changes were observed on the complex dielectric function of the niobate ceramic after the poling. The study of the properties of these ferroelectric ceramics is important in view of possible applications in ultrasonic devices such as delay lines and transducers.