The orthorhombic-cubic phase transformation in PbO2 was investigated employing cubic-anvil-type high-pressure apparatus combined with an energy dispersive-type X ray diffraction system in the pressure-temperature region to 84 kbar and 600¿C. In situ X ray observations confirmed the stability of both the orthrohombic columbite-type and the cubic fluorite-type polymorphs of PbO2. Considering the rate of transformation, the equilibrium phase boundary between PbO2 II (orthorhombic structure) and PbO2 III (cubic fluorite structure) was determined as P(kbar)=69-0.019T(¿C). A negative slope was established.