The high pressure and high temperature structural investigations on a and b phases of HfMo₂O₈ are reported. a-HfMo₂O₈ transforms to a monoclinic (d) phase, followed by a second transition to a triclinic (e) phase at 1.30(5) and 2.70(5) GPa, respectively. a-HfMo₂O₈ undergoes pressure induced amorphization above 24 GPa due to kinetically hindered phase transition to its high density polymorph b phase. The b-HfMo₂O₈ also undergoes a complete pressure induced amorphization above 35 GPa due to the kinetic hinderance to decomposition. When annealed at ambient pressure, b-HfMo₂O₈ reconverts to the trigonal  a phase above 700^oC.