Reactions between HgO, PbO, or PbO2 and 2.5-95 wt.% H2SO4 are studied at temperatures up to the boiling point of the acid. Depending on the oxide reactant, the H2SO4 concentration, and synthesis temperature, HgSO4, Hg3O2(SO4), PbSO4 and Pb2O(SO4) are obtained as identified reaction products. The thermal stability of HgSO4, Hg3O2(SO4), PbSO4, Pb2O(SO4), and PbO2 is examined and the results supplement and modify earlier findings. The redetermined crystal structure of Hg3O2(SO4) Oil the basis of powder neutron diffraction data shows that its space group (P3(1)) is of lower symmetry than earlier reported (P3(1)2(1)).