High pressure and high temperature quench experiments on α-MnO₂,Mn₂O₃ and sol gel derived manganese oxides have been carried out to identify any new phases to which the materials may transform under high pressure and high temperature conditions. Results of ESR, DTA and TGA investigations on sol gel derived manganese oxide have shown it to be hausmannite Mn₃O₄, instead of α-Mn₂O₃ as reported earlier in the lit-erature. The sol gel derived manganese oxide transforms to α-Mn₂O₃ when heated above 700^°C. Sol gel derived Mn₃O₄, when quenched from 5 GPa and temperature range 800–1200^°C, gives a mixture of Mn₃O₄ (hausmannite) and a phase having CaMn₂O₄(marokite)-type structure α-MnO₂ undergoes partial amorphi-zation when pressure-quenched from 8 GPa at room temperature. The high pressure and high temperature quench experiments up to 5 GPa and 700^°C showed that the decomposition temperature of α-MnO₂ increases with pressure. The new phase reported by Liu (1976) from diamond-anvil cell (DAC) experiments on pyrolusite MnO₂ is identified to be a low-density polymorph α-MnO₂ . This unusual result of formation of low-density α–MnO₂, having an open structure at high pressure and high temperature, is probably due to quenching of a non-equilibrium phase in Liu’s (1976) laser-heated DAC experiment.