An instrument for carrying out controlled microwave assisted chemical processes, and that is particularly useful for handling relatively small samples. The instrument includes a microwave-transparent reaction vessel with an open mouth, a pressure-resistant seal on the mouth of the vessel, and a needle, portions of which penetrate the seal with a first end of the needle and provide fluid communication into the vessel. A pressure transducer is at the opposite end of the needle and in fluid communication with the interior of the vessel through the needle. The instrument defines a pressure control flow path from a portion of the needle outside of the vessel to a fluid port, the flow path being in communication with the needle, the interior of the vessel and the transducer. A controllable pressure release valve for the flow path is associated with the port.