An instrument and method for high pressure microwave assisted chemistry are disclosed. The method includes the steps of applying microwave radiation to a sample in a sealed vessel while measuring the temperature and pressure generated inside the vessel and until the measured pressure reaches a designated set point, opening the vessel to release gases until the pressure inside the vessel reaches a lower set point, closing the vessel, and repeating the steps of opening the vessel at designated pressure set points and closing the vessel at designated pressure set points to the sample until the sample reaction reaches a designated high temperature. Microwave energy can be applied continuously or intermittently during the opening and closing steps. Among other items, the instrument includes pressure and temperature sensors and means for opening and closing the vessel at the set points.