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 + NaClO3). To illustrate the technical challenges the method overcomes, we applied this new method for two known pentavalent A(I)B(V)O3 postperovskite compounds. We successfully synthesized the two postperovskites, NaOsO3 and NaIrO3, and quenched to ambient conditions. Furthermore, we report the discovery of a new low-pressure polymorph of NaIrO3, 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.