Abstract: In the most preferred form, a microwave/steam sterilizer and its method of operation are disclosed including a waveguide having a waveguide conduit including a terminal aperture located one quarter wavelength from the bottom of a water reservoir and including a series of subapertures spaced in the range of one quarter wavelength from each other. The waveguide further includes a mitered corner moveable to shift the microwave standing waves to communicate with the terminal aperture and/or the series of subapertures in response to conditions sensed in the working cavity. The waveguide and the walls of the working cavity are angled with respect to each other to avoid reflection of microwave energy back to the microwave energy generator. The sterilizer hydrates potential pathogens and subjects them to relatively uniform electromagnetic energy without arcing and without self-destruction of the microwave source by reflected microwave energy.