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A TG and DTA investigation, under a static air atmosphere, of mixtures of •-MnO2 and Na2S~Os or K2S208 in different molar ratios is reported, fl-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 semiconductor fl-MnO2 should be used for the best activity.Stoichiometric molar ratios of 1:1 are established for the formation of Na2Mn(SO~)z and K2Mn2(SO~) 3. These salts are thermally stable from ambient temperature to 940 ~ . Reaction products are identified by X-ray diffraction.The reactions between alkalimetalpersulfates and metallic oxides have been studied with a derivatograph [1][2][3][4] by several authors with many aims.Barbooti and Jasim [1 -3] used oxides of Cu(ll), Ni(lI), Zn(I1) and Ti(IV), while Jaffer and Jasim [4] worked with oxides of Cr(II1), Fe(lIl), AI(IlI) and Cd(lI). The products of the thermal reactions are oxygen, sulfur trioxide, polymeric unstable intermediates and stable alkali metal compounds.Lunenko and co-workers [5] suggested a mechanism for persulfate-pyrosulfate transformation via formation of a pair of radical ions (SO4).The kinetic parameters [1] of conversion of sodium and potassium persulfates into the corresponding pyrosulfates have been determined too. Studies on the thermal characteristics of the effects of/3-MnO2 on the decomposition reactions of persulfates, using TG and DTA techniques, have not yet been reported.In our present work, we describe some studies giving new information about the reactions in the solid phases between ]?-MnO2 and sodium or potassium persulfate, and postulate the type of semiconductivity this oxide has under the described conditions. ExperimentalAnalytical grade salts were used. The fi-MnO2 employed as the other reactants with a minimum fl-MnO2 content of 80 %.Chemical and physical changes were studied as a function of temperature by means of a Paulik-Paulik-Erdey, MOM, derivatograph in air atmospheric pressure from room temperature up to 1000 ~