A TG and DTA investigation, under a static air atmosphere, of mixtures ofβ-MnO2 and Na2S2O8 or K2S2O8 in different molar ratios is reported.β-MnO2 lowers the initial decomposition temperatures of these persulfates by 25° through a catalytic effect. A mechanism is proposed for this effect. The X-ray diffractometry results in this investigation could be used to decide which type of semiconductorβ-MnO2 should be used for the best activity.