Abstract: A portable apparatus which produces high temperature and pressure steam derived from a microwave energy source for the rapid sterilization of medical and dental appliances. Items to be sterilized are placed in a detachable high pressure chamber which is then attached to the microwave powered steam source. Microwave energy is coupled into a guiding structure containing a shaped impedance transformation element so that essentially all of the microwave energy is transferred to a reaction fluid contained in a holder at the end of the matching element. The reaction fluid is rapidly vaporized and the resulting vapors rapidly expand into the chamber through a metal screen that also prevents transmission of microwave energy. Pressure temperature values internal to the chamber are used to control application of microwave power thereby maintaining an optimum micro-organism kill condition for a prescribed time interval.