This invention presents a high-pressure high-temperature reaction vessel having a metallized graphite heater for improved performance. The metallized heater consists of a graphite tube comprising a refractory metal. Metallization of the graphite heater is accomplished either by coating the graphite tube with a refractory metal, by intermixing a refractory metal in the graphite, or by positioning a refractory metal sheet or cylinder adjacent the graphite heater. The refractory metal constrains the graphite heater, adds toughness to the heater, shields the heater from outside contamination, and provides an electrical contact for a thermocouple.