A theoretical and experimental study was made of the rectification of microwave energy by a thin piezo‐electric disk placed in a ∼3‐GHz reentrant cavity. Using a one‐dimensional approximation, an equivalent circuit consisting of a frequency‐dependent voltage source in series with a capacitance was derived for the rectified output. The voltage is the sum of a coherent part, proportional to the nonlinear coefficients of the constitutive relations and to the Maxwell—Faraday stress, and a thermal part, proportional (for quartz) to the coefficient of thermal expansion. The risetime for the coherent voltage at an overtone acoustic resonance frequency was about 1 &mgr;sec, while that for the thermal heating was of the order of milliseconds. The rectified voltage is proportional to the microwave power and is typically mV/W when polished X‐cut disks are excited at acoustic resonance frequencies and one to two orders of magnitude smaller at other frequencies. No acoustic resonance was observed for C‐cut zinc oxide, and a nearly frequency‐independent value of 20 mV/W was measured.