Three binary systems, each consisting of mixtures of different molar ratios of a rare earth (4f metal) sesquioxide and sodium or potassium persulfate (peroxodisulfate) have been investigated under static (air) atmospheres using a derivatograph. For each system (R2O3 - M2S2O8, whereR is Sm, Gd, or Dy, andM is K or Na), the molar ratios that represent stoichiometric reactions were fixed. Calculations were based on data obtained from T, TG, DTG and DTA curves. Intermediate and final products were mainly identified by X-ray diffractometry. Double salts of Sm, Gd, and Dy were thermally prepared by solid-state reactions. It was found that one such double salt KDy(SO4)2, forms a eutectic mixture with K2SO4 and also that both Gd and Dy sesquioxides behave asp-type semiconductors and therefore exhibit catalytic activities towards the thermal decomposition of persulfates and pyrosulfates. Optimal catalysis was obtained with molar ratios of 1∶3. The activities increased as the ionic radii of these metals decrease: Sm, Gd, Dy.