In this paper, the frequency behaviour of electro-mechanical properties of PZN-xPT single crystals with compositions close to the morphotropic phase boundary is studied at frequencies up to 100MHz. Properties are deduced from the fit of the electrical input impedance, theoretically calculated with the KLM equivalent circuit model, to experimental data. To measure the electrical impedance of a crystal, its surfaces have first to be metallized. For very thin samples, this metallization may influence the resonances and alter the property identification. A theoretical study is thus performed to quantify the influence of the electrodes on the crystal vibrations. The results show that, even for high frequencies, their effects are negligible. In a second part, the evolution with frequency of the single crystal characteristics is studied. The properties are obtained in a frequency bandwidth centered on the resonances, from the fundamental to the 21(th) harmonic. The study shows the stability of the electromechanical properties versus the frequency. Finally, the mechanical and dielectric loss tangents are quantified.