Communication Obtaining single-phase Hg1-xMxBa2Ca2Cu3O8+delta materials has proved difficult using the sealed quartz tube technique, mainly because of the poor knowledge of the synthesis mechanisms. A technique is described for the measurement of the system pressure in situ, allowing both the thermodynamic and kinetic aspects of the reaction to be investigated. The power of this technique is demonstrated by studying the stability of the Hg0.82Re0.18Ba2Ca2Cu3O8+delta compound, but should also be applicable to insitu pressure determination in gas-solid reactions.