The gases evolved during operation of high power microwave devices, such as virtual cathode oscillators (vircators), can be detrimental to their power output, output pulse width, and maximum pulse repetition rate. Gases are evolved both from processes at the cathode (such as explosive electron emission) and at the anode (due to heating from the electron beam). In this project, a residual gas analyzer and pressure measurements have been utilized to characterize the gases generated during operation of a sealed vircator tube. The background pressure in the tube before firing is in the ultra-high vacuum range (∼10-8 Torr). The vircator is not pumped on during firing. After firing, the measurements are made, and then the tube is ion pumped back down to the background pressure before the next shot. Multiple anode and cathode materials will be tested, with measurements made of both the quantity and types of gases evolved during firing. Example test materials include stainless steel, OFHC copper, copper tungsten, and tantalum. For all tests, the vircator is driven directly from a low impedance Marx generator, with no intermediate pulse forming. The system design and diagnostic systems are described, and gas quantity and composition data is presented.