We report a new in situ synthesis method effective for discovery of high-oxidation-state materials using laser-heated diamond anvil cells. The issue of chemical reduction during thermally induced phase transitions that occur spontaneously in a noble gas pressure transmitting media (PTM) can be overcome by thermal decomposition of an oxygen-rich solid PTM (NaCl + NaClO 3 ). To illustrate the technical challenges the method overcomes, we applied this new method for two known pentavalent A (I) B (V) O 3 postperovskite compounds. We successfully synthesized the two postperovskites, NaOsO 3 and NaIrO 3 , and quenched to ambient conditions. Furthermore, we report the discovery of a new low-pressure polymorph of NaIrO 3 , illustrating the high potential for new materials discovery. This new method will enable realization of new high-oxidation-state postperovskites and can be applied for many other structure families in a P, T parameter space that is not easily accessible using conventional high-pressure synthesis methods.