Solid state reactions of CeO2, PuO2 and mixed oxides (U,Ce)O2 and (U,Pu)O2 containing different mol.% of Ce and Pu, were carried out with K2S2O8 at different temperatures to identify the formation of various products and to investigate their dissolution behaviour. X-ray, chemical and thermal analysis methods were used to characterise the products formed at various temperatures. The products obtained by heating two moles of K2S2O8 with one mole each of CeO2, PuO2, (U,Ce)O2 and (U,Pu)O2 at 400 °C were identified as K4Ce(SO4)4, K4Pu(SO4)4, K4(U,Ce)(SO4)4 and K4(U,Pu)(SO4)4, respectively. K4Ce(SO4)4 further decomposed to form K4Ce(SO4)3.5 at 600 °C and mixture of K2SO4 and CeO2 at 950 °C. Thus the products formed during the reaction of 2K2S2O8 + CeO2 show that cerium undergoes changes in oxidation state from +4 to +3 and again to +4. XRD data of K4Ce(SO4)4 and K4Ce(SO4)3.5 were indexed on triclinic and monoclinic system, respectively. PuO2 + 2K2S2O8 reacts at 400 °C to form K4Pu(SO4)4 which was stable upto 750 °C and further decomposes to form K2SO4 + PuO2 at 1000 °C. The products formed at 400 °C during the reactions of the oxides and mixed oxides were found to be readily soluble in 1–2 M HNO3.