For the purpose of quantitative evaluation of mechanochemical activation, stress was applied on fine powdered β-PbO 2 through vibro-milling and pressing. The degree of activation was evaluated thermoanalytically in terms of the change in the enthalpy of reduction into PbO. The energy supplied to the material was measured using the principle of isoperibolic calorimetry for the vibro-milling and from the stress—strain curve for the pressing. Increases in the enthalpy, ΔH*, up to 120 J/g and 24 J/g, through vibro-milling and pressing, respectively, were observed. The maximum effectivity of activation, i.e. the ratio ΔH* to the supplied energy was 4.5% for vibro-milling and 32% for pressing. A significant amount of mechanochemical phase transformation into a metastable α-phase was observed. The phase transformation alone is not sufficient, however, to explain the increase in the enthalpy. Rather, the total amount of crystallographic disturbance should be accounted for.