High Temperature piezoelectric single crystals ReCa4O(BO 3)3 -ReCOB (Re: rare earth element, such as Gd and Y) are readily grown by the Czochralski method. The melting temperature of these crystals was found to be around 1500°C, before which, no phase transitions occur. The dielectric constant and dielectric loss values for GdCOB were studied as a function of temperature. Electromechanical coupling characteristics of the GdCOB crystal were investigated for temperatures up to 1000°C, showing coupling keff up to 12% at room temperature with little change for temperatures up to 1000°C. High resistivities on the order of 108 Ω-cm at 700°C would contribute to low power requirements for electronics applications. The ability to operate these materials at high temperatures with stable dielectric and electromechanical coupling factors and low power requirements makes GdCOB an ideal candidate for temperature and resonant sensors in harsh environments.