The reactions during high-temperature decomposition of Ba(Zn1/2W1/2)O-3 double perovskite, a potential microwave dielectric material, were studied by using X-ray diffraction, electron microscopy, and Knudsen effusion combined with mass spectrometry. The results show that above 1200 degrees C, the perovskite decomposes due to the sublimation of ZnO, which results in the formation of BaWO4, Ba2WO5, and amorphous BaO-rich phases. The simultaneous presence of BaWO4 and Ba2WO5 causes the formation of a liquid phase above 1320 degrees C, which in the case of ceramics results in a progressive deterioration of the microstructure. As a consequence, the dielectric losses of Ba(Zn1/2W1/2)O-3-based ceramics strongly depend on the processing parameters.