Differential scanning calorimetry, differential thermogravimetry, X-ray analysis and electronic microscopic studies of thermal
transformations of PbO2 were carried out. Formation of fine dispersed (less than 100 nm) particles of α-PbO was observed at PbO2 thermal decomposition at heating to 580°C. Reverse reaction of Pb3O4 formation from PbO was found at cooling and annealing at 400°C in air. At heating of α-PbO to 650°C the particle growth to
1 μm with formation of β-PbO took place. Thermal decomposition with formation of β-PbO particles with size from 0.3 to 1 μm
at PbO2 heating to 650°C was observed. Transition from PbO to Pb3O4 at cooling of sample heated to 650°C was not detected. Interpretation of observed phenomena from the point of view of particle
size influences on the shift of α-PbO↔β-PbO phase transition temperature and on the chemical activity of phases are presented.